Allow docoments in toml to document crate features

[TimTheBig]

For a while documenting crate features has been quite the inconvenience, so a syntax along the lines of #!, in cargo.toml would be useful. To provide a more structured way to document features then putting the docs in your top-level docs.

[syphar]

Generally should be implemented in rustdoc itself, not in docs.rs.

There is already :

• an RFC nearly finished: RFC: Cargo feature descriptions rfcs#3485
• a tracking issue: Tracking Issue for Rustdoc support of RFC3485 feature-documentation rust#127771

for this feature, but no implementation yet.