h264_refs: make sure not to write over the bounds of the default ref list

Fixes invalid writes.
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org
h263dec: sanitize a condition.

Call ff_mpeg4_decode_picture_header() only when the decoder has the
MPEG4 codec id, not based on a vaguely related value of h263_pred.
doc/ffserver: document configuration syntax

While still incomplete, this is better than nothing at all.
ffserver: clarify feedback in case of wrong AVOptionAudio/Video error
doc/ffserver: remove outdated troubleshoot entry about audio
doc/default.css: sync with ffmpeg-web

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Merge commit '4d388c0cd05dd4de545e8ea333ab4de7d67ad12d'

* commit '4d388c0cd05dd4de545e8ea333ab4de7d67ad12d':
  h264_refs: make sure not to write over the bounds of the default ref list

Conflicts:
	libavcodec/h264_refs.c

This condition should be impossible and was checked for by asserts.
the asserts are moved up in this merge to gurantee that no out of array
access can happen even if the state is "impossible".
Also if such impossible states could somehow be created, that should
be dealt with and not silently ignored.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  h263dec: sanitize a condition.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples: add transcode_aac to local Makefile.
Add transcode_aac example to .gitignore.
doc/examples: make fill_samples static.

This is required to build with FFmpeg compilation options.
build: integrate remaining examples.
build: split examples cleaning out of docclean.
build: also clean the examples with debug symbols
doc/examples: update README.
doc/Makefile: cosmetics after previous commits.
doc/ffserver: remove more pointless/outdated entries

Mentioning build options, or bugs, or mysterious limitations should not
be done in the manual.
doc/ffserver: use @command{} for specifying ffserver
doc/ffserver: remove "What can this do?" section

The section shows only a limited use case, contains only redundant
information, and various mysterious mentions about things which don't
work correctly.
doc/ffserver: improve rendering for -n option text