Fix rendering of Contents when links are present

Fixes Haddock Trac #267.
Fix wording in the docs
Change rendering of duplicate record field docs

See Haddock Trac #195. We now change this behaviour to only rendering
the documentation attached to the first instance of a duplicate field.

Perhaps we could improve this by rendering the first instance that has
documentation attached to it but for now, we'll stick with this.
Render fixity information

Affects functions, type synonyms, type families, class names, data type
names, constructors, data families, associated TFs/DFs, type synonyms,
pattern synonyms and everything else I could think of.