Clarify behavior of ShardedHashMap::insert_unique

compiler/rustc_data_structures/src/sharded.rs

@@ -185,7 +185,9 @@ pub fn get_or_insert_with(&self, key: K, default: impl FnOnce() -> V) -> V
 
     /// Insert value into the [`ShardedHashMap`] with unique key.
     ///
-    /// Checks uniqueness if debug_assertions enabled.
+    /// This function panics if debug_assertions are enabled and uniqueness is violated.
+    /// If uniqueness is violated but debug_assertions are disabled then lookups will arbitrarily
+    /// return one of the inserted elements.
     #[inline]
     pub fn insert_unique(&self, key: K, value: V) {
         let hash = make_hash(&key);