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