Fix -mdoc markup: use .Fa for function arguments.
Mark up 'NULL' consistently in libdwarf's manual pages.
Use the recommended ordering for the '.Dd', '.Dt' and '.Os' macros.
Turn on SVN keyword expansion. Ticket: #348
Word a sentence more carefully.
Minor rework of the `dwarf_expr_current_offset.3` manual page.
Ticket: #228
Add manual page for dwarf_expr_current_offset.
