std: Add performance warnings to HashMap::get_disjoint_mut

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

Diff for: library/std/src/collections/hash/map.rs

@@ -973,6 +973,9 @@ where
     /// Returns an array of length `N` with the results of each query. For soundness, at most one
     /// mutable reference will be returned to any value. `None` will be used if the key is missing.
     ///
+    /// This method does a O(n^2) check to check that there are no duplicate keys, so be careful
+    /// when passing many keys.
+    ///
     /// # Panics
     ///
     /// Panics if any keys are overlapping.