Fix import in module_api module and docs on the new check_event_for_spam signature (#12918)

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

synapse/module_api/__init__.py

@@ -35,7 +35,6 @@ from typing_extensions import ParamSpec
 from twisted.internet import defer
 from twisted.web.resource import Resource
 
-from synapse import spam_checker_api
 from synapse.api.errors import SynapseError
 from synapse.events import EventBase
 from synapse.events.presence_router import (
@@ -55,6 +54,7 @@ from synapse.events.spamcheck import (
     USER_MAY_JOIN_ROOM_CALLBACK,
     USER_MAY_PUBLISH_ROOM_CALLBACK,
     USER_MAY_SEND_3PID_INVITE_CALLBACK,
+    SpamChecker,
 )
 from synapse.events.third_party_rules import (
     CHECK_CAN_DEACTIVATE_USER_CALLBACK,
@@ -140,9 +140,7 @@ are loaded into Synapse.
 """
 
 PRESENCE_ALL_USERS = PresenceRouter.ALL_USERS
-
-ALLOW = spam_checker_api.Allow.ALLOW
-# Singleton value used to mark a message as permitted.
+NOT_SPAM = SpamChecker.NOT_SPAM
 
 __all__ = [
     "errors",
@@ -151,7 +149,7 @@ __all__ = [
     "respond_with_html",
     "run_in_background",
     "cached",
-    "Allow",
+    "NOT_SPAM",
     "UserID",
     "DatabasePool",
     "LoggingTransaction",