rustdoc: Don't assume that a doc attribute was sugared: Fixes #15976

As of 8876ce44, `is_sugared_doc` is encoded in metadata, so there is no
need to assume that doc attributes came from sugared comments.
auto merge of #16766 : kevinmehall/rust/issue-15976, r=alexcrichton

As of 8876ce44, `is_sugared_doc` is encoded in metadata, so there is no
need to assume that all `doc` attributes came from sugared comments.

Fixes #15976