Previously pandoc would parse
[link to (@a)](url)
as a citation; similarly
[(@a)]{#ident}
This is undesirable. One should be able to use example references
in citations, and even if `@a` is not defined as an example
reference, `[@a](url)` should be a link containing an author-in-text
citation rather than a normal citation followed by literal `(url)`.
Closes#7632.
