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).
e1e4816e16