Correct check_username_for_spam annotations and docs (#12246)

* Formally type the UserProfile in user searches
* export UserProfile in synapse.module_api
* Update docs

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

synapse/types.py

@@ -34,6 +34,7 @@ from typing import (
 import attr
 from frozendict import frozendict
 from signedjson.key import decode_verify_key_bytes
+from typing_extensions import TypedDict
 from unpaddedbase64 import decode_base64
 from zope.interface import Interface
 
@@ -63,6 +64,10 @@ MutableStateMap = MutableMapping[StateKey, T]
 # JSON types. These could be made stronger, but will do for now.
 # A JSON-serialisable dict.
 JsonDict = Dict[str, Any]
+# A JSON-serialisable mapping; roughly speaking an immutable JSONDict.
+# Useful when you have a TypedDict which isn't going to be mutated and you don't want
+# to cast to JsonDict everywhere.
+JsonMapping = Mapping[str, Any]
 # A JSON-serialisable object.
 JsonSerializable = object
 
@@ -791,3 +796,9 @@ class UserInfo:
     is_deactivated: bool
     is_guest: bool
     is_shadow_banned: bool
+
+
+class UserProfile(TypedDict):
+    user_id: str
+    display_name: Optional[str]
+    avatar_url: Optional[str]