doc/filters: review introductory example and explanation

In particular, fix wrong vertical mirroring command, and clarify
and extend explanation.

Based on a patch by littlebat <dashing.meng@gmail.com>.

Should fix trac ticket #2413.
(cherry picked from commit 215ca864759a54f45265a51ac57dbfd75cb23da2)

Signed-off-by: Timothy Gu <timothygu99@gmail.com>

Conflicts:
	doc/filters.texi