Add contest performance ceiling to API

Author: Ninjaclasher

docs/site/api.md

@@ -139,8 +139,9 @@ Example: [/api/v2/contest/bts19](https://dmoj.ca/api/v2/contest/bts19)
     "is_rated": "<whether the contest is rated>",
     "rate_all": "<whether the contest is rated on join>",
     "has_rating": "<whether the contest has been rated>",
-    "rating_floor": "<the minimum user rating required for the user to be rated>",
-    "rating_ceiling": "<the maximum user rating for the user to be rated>",
+    "rating_floor": "<the minimum user rating required for the user to be rated, or null if there is no minimum>",
+    "rating_ceiling": "<the maximum user rating for the user to be rated, or null if there is no maximum>",
+    "performance_ceiling": "<the maximum user rating possible from this contest, or null if there is no maximum>",
     "hidden_scoreboard": "<whether the contest's scoreboard is hidden>",
     "scoreboard_visibility": "<whether the scoreboard is (V)isible, visible after (C)ontest, or visible after (P)articipation>",
     "is_organization_private": "<whether the contest is private to organizations>",