Fix -mdoc markup: use .Fa for function arguments.
Mark up 'NULL' consistently in libdwarf's manual pages.
Style fixes for the dwarf(3) manual pages, per 'mandoc -T lint'.
Use the recommended ordering for the '.Dd', '.Dt' and '.Os' macros.
Turn on SVN keyword expansion. Ticket: #348
Minor revisions to the `dwarf_add_AT_dataref.3` manual page.
Ticket: #217
Fix copy-n-paste error in RETURN VALUES section.
Add manual page for dwarf_add_AT_dataref.
