Allow intra doc link resolution failures, because they actually succeed when they need to but rust doesnt know

[rukai]

Description
cargo doc will create a lot of warnings due to the intra doc links pointing to wgpu.
The links work fine, but the warnings are no good because they show up when any any crate dependent on wgpu runs cargo doc.

Testing
Ran cargo doc from wgpu-types:

• before: warnings from this crate.
• after: no warnings from this crate.

Ran cargo doc from wgpu:

• before: warnings from this crate
• after: no warnings from this crate

[kvark]

cargo doc will claim all the links cannot be resolved
// despite the fact that it works fine when it needs to

Is this a cargo bug then? Would be good to link to it from somewhere

[rukai]

well when running cargo doc for wgpu-types, wgpu is not built so there is no way for the links to succeed here. Might be worth disabling the warnings for this case alone.

However I guess I should still make a cargo issue for the warnings to not appear when running cargo doc for wgpu.

[kvark]

bors r+

[bors]

Build succeeded:

• Ubuntu Stable
• MacOS Nightly
• iOS Stable
• MacOS Stable
• Android Stable
• Ubuntu Nightly
• Windows Nightly
• Windows Stable