rustdoc should support --cfg

[thestinger]

I use the libc configuration flag to isolate functionality unavailable for freestanding Rust in rust-core, and this means a lot of stuff is missing from the generated documentation.

[klutzy]

I actually hope something more: cfg-dependent items.
For example, std::path currently shows pub use self::posix::Path; but it would be better if it shows that Path is sometimes self::posix::Path and some other times self::windows::Path.

[emberian]

@klutzy that is the subject of #1998

[vhbit]

@cmr is handling of --cfg feature=some_feat a part of this issue or #1998?

[emberian]

@vhbit #1998