doc-rst: moved *duplicate* warnings to nitpicky mode

Moved the *duplicate C object description* warnings for function
declarations in the nitpicky mode. In nitpick mode, you can suppress
those warnings (e.g. ioctl) with::

  nitpicky = True
  nitpick_ignore = [
      ("c:func", "ioctl"),
  ]

See Sphinx documentation for the config values for ``nitpick`` and
``nitpick_ignore`` [1].

With this change all the ".. cpp:function:: int ioctl(..)" descriptions
(found in the media book) can be migrated to ".. c:function:: int
ioctl(..)", without getting any warnings. E.g.::

  .. cpp:function:: int ioctl( int fd, int request, struct cec_event *argp )

  .. c:function:: int ioctl( int fd, int request, struct cec_event *argp )

The main effect, is that we get those *CPP-types* back into Sphinx's C-
namespace and we need no longer to distinguish between c/cpp references,
when we refer a function like the ioctl.

[1] http://www.sphinx-doc.org/en/stable/config.html?highlight=nitpick#confval-nitpicky

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>