FAQ: TRIM: GELI, ZFS, OpenZFS: pruning, tidiness

The ViewVC views of 2013 Subersion revisions are no longer of interest.

vfs.zfs.trim.enabled is unknown in FreeBSD 13.1-RELEASE-p3.

Questions about TRIM in the context of GELI are not frequently asked.

Also, non-contentious grammar, punctuation, and formatting:

• surrounding apostrophes ('…') are obviously missing from some quotes
• missing full stops (.)
• trailing white space
• AsciiDoc line breaks
• et cetera, there might remain a superfluous break, such things can be addressed as a fix to this commit.

This commit reduces, but does not fix, bug report 261212:

Update the ZFS chapter (20) of the FreeBSD Handbook, and other
OpenZFS-related pages

PR: 261212
Reviewed by: mav, pauamma
Approved by: delphij (doc)
Differential revision: https://reviews.freebsd.org/D37512