docs: config.md updates

docs: config.md updates

The theories behind bees slowing down when presented with a larger has
table turned out to be wrong.  The real cause was a very old bug which
submitted thousands of `LOGICAL_INO` requests when only a handful of
requests were needed.

"Compression on the filesystem" -> "Compression in files"

Don't be so "dramatic".  Be "rapid" instead.

Remove "cannot avoid modifying read-only snapshots" as a distinction
between subvol and extent scans.  Both modes support send workaround
and send waiting with no significant distinction.

Emphasize extent scan's better handling of many snapshots.  Also reflinks.

Add some discussion of `--throttle-factor`.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>