Make cfg imply doc(cfg)

[Nemo157]

This is only active when the doc_cfg feature is active.

The implicit cfg can be overridden via #[doc(cfg(...))], so e.g. to hide a #[cfg] you can use something like:

#[cfg(unix)]
#[doc(cfg(all()))]
pub struct Unix;

By adding #![doc(cfg_hide(foobar))] to the crate attributes the cfg #[cfg(foobar)] (and only that exact cfg) will not be implicitly treated as a doc(cfg) to render a message in the documentation.

TODO:

• Test against some popular libraries, especially those already using doc(cfg)
• Update doc(cfg) docs
• Add doc(cfg_hide) docs