Fix some wording about errors

see https://github.com/rust-lang/rust/pull/18176#discussion_r19374679
Describe doc attribute in the reference

FIxes #7963.
auto merge of #18376 : steveklabnik/rust/gh7963, r=alexcrichton

FIxes #7963.
auto merge of #18377 : steveklabnik/rust/fix_wording_about_errors, r=nikomatsakis

see https://github.com/rust-lang/rust/pull/18176#discussion_r19374679

/cc @eddyb @huonw @nikomatsakis