rustdoc: display rustc_private APIs as "Internal"

[euclio]

This PR updates the display of rustc_private APIs to be "Internal" instead of "Experimental", and changes the colors appropriately. It also updates the copy of the rustc_private feature to sound more informative and less like a compiler suggestion.

The PR additionally contains a significant refactor of the short_stability function to remove duplication and fix a few rendering bugs due to extra or missing spaces.

Before:

After:

[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:00cf2c79:start=1544807335307619232,finish=1544807338304616753,duration=2996997521
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-5.0
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---
[00:47:46] 
[00:47:46] ---- [ui] ui/issues/issue-37887.rs stdout ----
[00:47:46] diff of stderr:
[00:47:46] 
[00:47:46] 4 LL |     use libc::*; //~ ERROR unresolved import
[00:47:46] 5    |         ^^^^ maybe a missing `extern crate libc;`?
[00:47:46] - error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
[00:47:46] - error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
[00:47:46] + error[E0658]: use of unstable library feature 'rustc_private': This crate is being loaded from the sysroot, a permanently unstable location for private compiler dependencies. It is not intended for general use. Prefer using a public version of this crate from [crates.io](https://crates.io) via [`Cargo.toml`](https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html). (see issue #27812)
[00:47:46] 9    |
[00:47:46] 9    |
[00:47:46] 10 LL |     extern crate libc; //~ ERROR use of unstable
[00:47:46] 
[00:47:46] The actual stderr differed from the expected stderr.
[00:47:46] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-37887/issue-37887.stderr
[00:47:46] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-37887/issue-37887.stderr
[00:47:46] To update references, rerun the tests and pass the `--bless` flag
[00:47:46] To only update this specific test, also pass `--test-args issues/issue-37887.rs`
[00:47:46] error: 1 errors occurred comparing output.
[00:47:46] status: exit code: 1
[00:47:46] status: exit code: 1
[00:47:46] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/issues/issue-37887.rs" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-37887/a" "-Crpath" "-O" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/issues/issue-37887/auxiliary" "-A" "unused"
[00:47:46] ------------------------------------------
[00:47:46] 
[00:47:46] ------------------------------------------
[00:47:46] stderr:
[00:47:46] stderr:
[00:47:46] ------------------------------------------
[00:47:46] {"message":"unresolved import `libc`","code":{"code":"E0432","explanation":"\nAn import was unresolved.\n\nErroneous code example:\n\n```compile_failst" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/ui" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-5.0/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python2.7" "--lldb-python" "/usr/bin/python2.7" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "5.0.0\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--llvm-cxxflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"
[00:47:46] 
[00:47:46] 
[00:47:46] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[00:47:46] Build completed unsuccessfully in 0:03:51
[00:47:46] Build completed unsuccessfully in 0:03:51
[00:47:46] make: *** [check] Error 1
[00:47:46] Makefile:58: recipe for target 'check' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_tFri, 14 Dec 2018 17:56:53 GMT
travis_time:end:04ec2a7e:start=1544810213363188482,finish=1544810213401227658,duration=38039176
The command "date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[Dylan-DPC-zz]

r? @QuietMisdreavus

[QuietMisdreavus]

This color seems a bit dark to me, somehow. The color-contrast tool i fed the colors to said it should be fine, but i think it would be much easier to read if the background color here (and in the light theme) were changed to something like #ffb9b3. That gives you something like this:

[euclio]

Yeah, that color looks better to me too. Fixed.

[QuietMisdreavus]

Thanks so much! I like the refactors you did, it makes the code easier to read.

[QuietMisdreavus]

@bors r+

[bors]

📌 Commit 8d393bf has been approved by QuietMisdreavus

[bors]

⌛ Testing commit 8d393bf with merge 24667aa...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: QuietMisdreavus
Pushing 24667aa to master...