Add "Returns:" field list syntax to QAPI Commands.

Like "Arguments:" and "Errors:", the type name isn't currently processed
for cross-referencing, but this will be addressed in a forthcoming
commit.

This patch adds "errors" as a GroupedField, which means that multiple
return values can be annotated - this is only done because Sphinx does
not seemingly (Maybe I missed it?) support mandatory type arguments to
Ungrouped fields. Because we want to cross-reference this type
information later, we want to make the type argument mandatory. As a
result, you can technically add multiple :return: fields, though I'm not
aware of any circumstance in which you'd need or want
to. Recommendation: "Don't do that, then."

Since this field describes an action/event instead of describing a list
of nouns (arguments, features, errors), I added both the imperative and
indicative forms (:return: and :returns:) to allow doc writers to use
whichever mood "feels right" in the source document. The rendered output
will always use the "Returns:" label, however.

I'm sure you'll let me know how you feel about that. O:-)

Signed-off-by: John Snow <jsnow@redhat.com>
---
 docs/sphinx/qapi-domain.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/docs/sphinx/qapi-domain.py b/docs/sphinx/qapi-domain.py
index 4bfc82aafdf..c8c8ffc0dce 100644
--- a/docs/sphinx/qapi-domain.py
+++ b/docs/sphinx/qapi-domain.py
@@ -268,6 +268,13 @@ class QAPICommand(QAPIObject):
                 names=("error", "errors"),
                 has_arg=False,
             ),
+            # :returns TypeName: descr
+            GroupedField(
+                "returnvalue",
+                label=_("Returns"),
+                names=("return", "returns"),
+                can_collapse=True,
+            ),
         ]
     )
 
-- 
2.47.0