API Docs: fmt

[steveklabnik]

Part of #29329

http://doc.rust-lang.org/std/fmt/

Here's what needs to be done to close out this issue:

• the module level docs almost belong in their own issue. These are very old docs written long ago, and while they're good, I have a hunch they could be made way better. Not needed to close this issue.
• all of the traits should say "C formatting", where C is the char, rather than their current boilerplate. Except Write. And Display.
• fmt::Result needs an example, showing off the use std::fmt; fmt::Result pattern.
• write should have its Arguments section removed, we don't do this.
• format has the same issue, and needs more explanation generally.
• All of the traits have... I dunno, the wording feels awkward. This doesn't need to be fixed to close this issue, but would be nice.
• Write needs a lot of links. and some examples.
• Arguments needs examples, but also a justification of why this is all done this way.
• DebugList should use the iterator boilerplate language, even though it's not an iterator. Mostly pointing to the docs of the function that creates it.
• DebugMap same.
• DebugSet same.
• DebugStruct same.
• DebugTuple same.
• Error needs examples, and specifically use std::fmt; fmt::Error
• Formatter should show where it's created, and talk about the details more.