doc: remove mention of fifo filter in the introduction.

It's confusing and not necessary, especially in the introduction.
(cherry picked from commit 80a9297b711b7acb152ebbf84b28772945f920ea)

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
doc/filters: remove outdated comments

Audio API should not be considered unstable any longer, and mentioning
VHOOK is confusing, as it was removed 4 years ago.
(cherry picked from commit 86c6bf040bd3a1f7444c32a7af550edef2dfc2c4)

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