docs: Fix gap following symbols without a prototype.
docs: Fix synopsis section in man pages.

Prototype lines have no leading indentation when loaded since the
highlighting change, which was expected by the man page writer.
docs: Update for Pandoc cross-reference changes.

The handling of cross-references in LaTeX and Texinfo writers changed in
more recent versions of Pandoc (1.10 or thereabouts) which broke our output.
Update our code accordingly.  Drop support for older Pandoc versions.

The dummy_refs hack is no longer required.
Drop the WANT_DOCS_PDF_PAPER option as a side-effect.
docs: Delete considerations for older Pandoc.