qapi: use imperative style in documentation

As requested by Markus:
> We prefer imperative mood "Return" over "Returns".

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-ID: <20250522190542.588267-14-pierrick.bouvier@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Change several more]
12 files changed