Fix UserWarning on Flask 2.2.0

Flask 2.2.0 issues a warning if a url rule is added to a blueprint after it
has been registered, so this commit ensures that the documentation rules
are added before the blueprint is registered.

Fixes: python-restx#485

Signed-off-by: Jonathan Dieter <jonathan.dieter@spearline.com>

flask_restx/api.py

@@ -233,6 +233,8 @@ def init_app(self, app, **kwargs):
         # If app is a blueprint, defer the initialization
         try:
             app.record(self._deferred_blueprint_init)
+            self._register_specs(app)
+            self._register_doc(app)
         # Flask.Blueprint has a 'record' attribute, Flask.Api does not
         except AttributeError:
             self._init_app(app)
@@ -245,9 +247,6 @@ def _init_app(self, app):
 
         :param flask.Flask app: The flask application object
         """
-        self._register_specs(self.blueprint or app)
-        self._register_doc(self.blueprint or app)
-
         app.handle_exception = partial(self.error_router, app.handle_exception)
         app.handle_user_exception = partial(
             self.error_router, app.handle_user_exception