add documentation for "template" parameter in postgresql_db

postgresql_db

@@ -49,6 +49,11 @@ options:
       - Name of the role to set as owner of the database
     required: false
     default: null
+  template:
+    description:
+      - Template used to create the database
+    required: false
+    default: null
   state:
     description:
       - The database state