rustdoc: whitelist the headers that get a § on hover.

rustdoc: whitelist the headers that get a § on hover.

Previously the :hover rules were making the links to the traits/types in
something like

    impl<K: Hash + Eq, V> ... { ... }

be displayed with a trailing `§` when hovered over. This commit
restricts that behaviour to specific headers, i.e. those that are known
to be section headers (like those rendered in markdown doc-comments, and
the "Modules", "Functions" etc. headings).