Collapse multiple retvals into a single bullet list

[jakobandersen]

After the change to use Docutils/Sphinx field lists the rendering of \retval was a bit icky as there is no direct equivalent.
This changes the rendering of them to gather them into a single field. If there are multiple \retvals they will be made into a bullet list in the same field body.
E.g., for

/*! Something myFunction short.
 *
 * Something myFunction long.
 *
 * @param p pdesc
 * @param fp fpdesc
 * @return ReturnDesc
 * @retval "Discovery status" The discovery status
 * @retval MAX_EVENTS         The pointer
 */
void myFunction(int *p, void (*fp)()) {

it now gets compiled into

Note, only the \revals are touched, as they are the ones that mentions a value, but \returnss are left as individual fields as map directly to the field in Sphinx (is it common to have both \retvals and \retuns at the same time?)

See also #684 and #687.
Fixes part of #695.

[vermeeren]

(is it common to have both \retvals and \retuns at the same time?)

@jakobandersen Specifically about this, I haven't seen this in the wild myself, so should be fine as-is. Could be I have experience with the wrong projects though.

[vermeeren]

Thanks!