Add `generator_class` parameter to docs.

`get_schema_view` accepts `generator_class` parameter

docs/api-guide/schemas.md

@@ -185,6 +185,12 @@ to be exposed in the schema:
         patterns=schema_url_patterns,
     )
 
+#### `generator_class`
+
+May be used to specify a `SchemaGenerator` subclass to be passed to the
+`SchemaView`.
+
+
 
 ## Using an explicit schema view