renderer: use retval type for the retval fields

[gmarull]

This change makes @retval fields to render as a separate list, independent of the @return/s entries.

Depends on sphinx-doc/sphinx#9691

Example:

/**
 * @brief Foo.
 *
 * @param[in] bar Bar
 *
 * @retval 0 If succeeded
 * @retval 1 If something went wrong
 */
int foo(int bar);

[jakobandersen]

Good idea, though:

• This will only go well in general if the C++ domain of Sphinx gets the same feature, so I made C++, add retval info field sphinx-doc/sphinx#9694.
• To avoid bumping the minimum version requirement of Sphinx to 4.3 I suggest making this conditional on the Sphinx version. I.e.,

        if sphinx.version_info[0:2] < (4, 3):
            fieldListName = {
                "param": "param",
                "exception": "throws",
                "templateparam": "tparam",
                "retval": "returns",
            }
        else:
            fieldListName = {
                "param": "param",
                "exception": "throws",
                "templateparam": "tparam",
                "retval": "retval",
            }

Similarly, the hax to collect retvals (the whole conditional code at https://github.com/michaeljones/breathe/blob/327374db4cc440bec39dd4c6f96471035aa56317/breathe/renderer/sphinxrenderer.py#L899) should be disabled if the Sphinx version is at least 4.3. While I don't think it does harm, it's nice to have the version check so it's clear it should be deleted at some point.

[gmarull]

Thanks for the tips @jakobandersen, I've updated the patch.

[gmarull]

@michaeljones ping

[jakobandersen]

Looks good, thanks!