ZFS + TRIM + GELI documentation update

[Eric A. Borisch]

Documentation team,

Please update https://www.freebsd.org/doc/faq/all-about-zfs.html --
specifically the note in section 9.7.

GELI support for TRIM was added to CURRENT with r286444; this is also
in stable/11.

The note should be updated to reflect this, and to state that for
versions prior to these, TRIM requests are (safely, but potentially
with different performance characteristics) rejected on GELI devices.

Thanks for your hard work!
  - Eric