Generate missing docs for out arguments

The reStructuredText generator is missing the documentation blocks for
out arguments.

gio/gdbus-2.0/codegen/codegen_rst.py

@@ -231,6 +231,13 @@ class RstCodeGenerator:
                     f"  {arg_desc}",
                     "",
                 ]
+            for a in m.out_args:
+                arg_desc = self._expand(a.doc_string, True)
+                res += [
+                    f"{a.name}",
+                    f"  {arg_desc}",
+                    "",
+                ]
             res += [""]
             if m.since:
                 res += [