Clarify that ordering is unspecified

Author: digama0

library/alloc/src/slice.rs

@@ -373,6 +373,8 @@ impl<T> [T] {
     ///
     /// During sorting, the key function is called at most once per element, by using
     /// temporary storage to remember the results of key evaluation.
+    /// The order of calls to the key function is unspecified and may change in future versions
+    /// of the standard library.
     ///
     /// This sort is stable (i.e., does not reorder equal elements) and *O*(*m* \* *n* + *n* \* log(*n*))
     /// worst-case, where the key function is *O*(*m*).