Functions and data structure cross references with Sphinx

[Mauro Carvalho Chehab <mchehab@s-opensource.com>]

There's one remaining major issue I noticed after the conversion of the
media books to Sphinx:

While sphinx complains if a cross-reference (using :ref:) points to an
undefined reference, the same doesn't happen if the reference uses
:c:func: and :c:type:.

In practice, it means that, if we do some typo there, or if we forget to
add the function/struct prototype (or use the wrong domain, like :cpp:),
Sphinx won't generate the proper cross-reference, nor warning the user.

That's specially bad for media, as, while we're using the c domain for
the kAPI and driver-specific books, we need to use the cpp domain on the 
uAPI book - as the c domain doesn't allow multiple declarations for
syscalls, and we have multiple pages for read, write, open, close, 
poll and ioctl.

It would be good to have a way to run Sphinx on some "pedantic"
mode or have something similar to xmlint that would be complaining
about invalid c/cpp domain references.

Thanks,
Mauro