doc: improve match pattern for manual page references with custom_see_handler.

* doc/doc.md: Use '[%w_]+' instead of '%a+' so that references
to, say, _exit(2) and dup2(2) work too.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
Merge pull request #129 from gvvaughan/pull-request/custom_see_hander-doc-fix

doc: improve match pattern for manual page references with custom_see_handler.