qapi: Add missing union tag documentation

Low-hanging fruit, and except for StatsFilter, the only members of
these unions lacking documentation.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240205074709.3613229-16-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
diff --git a/qapi/transaction.json b/qapi/transaction.json
index cffee2d..7a95c08 100644
--- a/qapi/transaction.json
+++ b/qapi/transaction.json
@@ -158,6 +158,8 @@
 # A discriminated record of operations that can be performed with
 # @transaction.
 #
+# @type: the operation to be performed
+#
 # Since: 1.1
 ##
 { 'union': 'TransactionAction',