doc: Fix a few typos in the developer documentation

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
doc: Merge disjointed bits about emailing patches

We obviously prefer git-send-email(1), and the disjointed nature
of the two statements was misleading.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
doc: Mention zzuf in the fuzz testing section

It's pretty much the best tool for the job, and it's what we
all use now anyway.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
doc: Mention memory allocation in the fuzz testing section

It's obviously undesireable to blindly allocate memory based on
a damaged 'size' value, for example.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
doc: Extend commit message section

If a bug exists on the tracker, its ID should always be included
in fix messages.

Also, any relevant bug fixes should be CC'd to libav-stable, so
we can actually track what needs to be backported, instead of
just randomly combing the git history and old CVEs.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>