std: Add performance warnings to HashMap::get_disjoint_mut

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

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 performs a check to ensure there are no duplicate keys, which has time-complexity O(n^2),
+    /// so be careful when passing many keys.
+    ///
     /// # Panics
     ///
     /// Panics if any keys are overlapping.