[media] doc-rst: Don't use captions for examples

Unfortunately, captions are new on Sphinx for c blocks: it was
added only on version 1.3. Also, it were already bad enough
not being able to auto-numerate them.

So, let's give up and use, instead, titles before the examples.
Not much is lost, and, as a side track, we don't need to
numerate them anymore.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>