Fix retval rendering

[jakobandersen]

Doxygen has both \returns desc and \retval arg desc which is converted to the equivalent of :returns: desc and :returns arg: desc. As noted in #683 (comment), all the build-in Sphinx domains assumes the returns field to have no argument, both it still handles them. Except, it must apparently be a single docutils node. This PR makes it that.

Example for reproduction:
test.hpp

/*! Something myFunction short.
 *
 * Something myFunction long.
 *
 * @retval "Discovery status" - The discovery status
 * @retval MAX_EVENTS         - The pointer
 * @return ReturnDesc
 */
void myFunction();

index.rst:

.. doxygenfunction:: myFunction

.. cpp:function:: void f()

	:returns: something
	:returns arg: somethingElse

Fixes #684.

[vermeeren]

@jakobandersen Nice one, thanks!