stand/lua manuals: Describe better

The lua boot loader module manuals were getting pulled into search
results for FreeBSD, but not for "boot" or "loader". Reword them to
increase clarity for boot or loader searches, as well as the FreeBSD
search term which we've been scoping to system topic overview manuals.

MFC after: 3 days
Reviewed by: imp, mhorne
Approved by: mhorne (mentor)
Pull Request: https://github.com/freebsd/freebsd-src/pulls/1628