status/report-template.xml: Improve Documentation category description

According to its description, the Documentation category includes only the
doc tree and external documentation. This commit explicitly adds manpages,
so that reports involving manpages in the base or ports tree can also be
categorized under Documentation. While here, it also adds the possibility
to include reports modifying existing external documenation.

Approved by: bcr (docs), gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D24305