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)
Approved by: re (cperciva)
Pull Request: https://github.com/freebsd/freebsd-src/pulls/1628

(cherry picked from commit 5d59c1b4f14e2fa6388b1d75ee3243085f1c69f2)
(cherry picked from commit 651102873e96174e4e799d4e06538e1d841e182a)