quot: Improve manual page

• Rewrite the first paragraph to clarify that quot(8) only supports FFS.

• Document which options can or cannot be combined with each other.

MFC after: 1 week
Reviewed by: ziaee
Differential Revision: https://reviews.freebsd.org/D53727

(cherry picked from commit 2f3adab9d512faec3e1891e505d1888db5d0240e)