Rustdoc: Surface trait impl doc comments in "implemented" section

[sfackler]

rust-postgres is going to be providing more built-in implementations of ToSql and FromSql for third part libraries via Cargo features (e.g. sfackler/rust-postgres@4f80c32). I've tagged the trait impls with a doc comment stating which feature is required to use them (sfackler/rust-postgres@b81acaa), but that comment isn't displayed by rustdoc: http://sfackler.github.io/doc/postgres/trait.ToSql.html.

[sfackler]

cc @alexcrichton

[jan-hudec]

Note that the comments do appear in the “Trait implementations” section of the type's documentation. But for libraries that define new traits and implement them for existing types this is useful. The comments have to also appear in the “Implementors” section trait's documentation because depending on whether new type or new trait is being created the crate may be only generating one of these pages.

[sfackler]

This has been a persistent source of confusion for users of rust-postgres:
sfackler/rust-postgres#121
sfackler/rust-postgres#120
sfackler/rust-postgres#112
sfackler/rust-postgres#103
sfackler/rust-postgres#102

[steveklabnik]

Triage: no change.