All sub-classes of QAPISchemaEntity now override .check_doc() the same
way, except for QAPISchemaType and and QAPISchemaArrayType.
Put the overrides' code in QAPISchemaEntity.check_doc(), and drop the
overrides. QAPISchemaType doesn't care because it's abstract.
QAPISchemaArrayType doesn't care because its .doc is always None.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191024110237.30963-14-armbru@redhat.com>
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