mount: Improve usage message and polish man page descriptions

+ license: tag spdx, remove dead hyphen
+ -t: fix linter errors, sync to program usage
+ -o ro: explain itself before explaining it's equivalence
+ -o rw: mention

MFC after: 1 week
Fixes: 70866c8dbdb6c (Fix synopsis of the -t option)
Reviewed by: imp, mhorne, Pat Maddox <pat@patmaddox.com>
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D49671