Update API docs to use 'presence' key not 'state'. Fixed error messages when setting presence.

docs/client-server/swagger_matrix/api-docs-presence

@@ -106,7 +106,7 @@
     "PresenceUpdate": {
       "id": "PresenceUpdate",
       "properties": {
-        "state": {
+        "presence": {
           "type": "string",
           "description": "Enum: The presence state.",
           "enum": [

synapse/handlers/presence.py

@@ -207,7 +207,7 @@ class PresenceHandler(BaseHandler):
             raise SynapseError(400, "User is not hosted on this Home Server")
 
         if target_user != auth_user:
-            raise AuthError(400, "Cannot set another user's displayname")
+            raise AuthError(400, "Cannot set another user's presence")
 
         if "status_msg" not in state:
             state["status_msg"] = None