renderer: use retval type for the retval fields

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

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

breathe/renderer/sphinxrenderer.py

@@ -2211,7 +2211,7 @@ def visit_docparamlist(self, node) -> List[Node]:
             "param": "param",
             "exception": "throws",
             "templateparam": "tparam",
-            "retval": "returns",
+            "retval": "retval",
         }
 
         # https://docutils.sourceforge.io/docs/ref/doctree.html#field-list