Attempt to clarify the deallocation rules for UMBP. (swiftlang#85101)

resolves rdar://162944521

---------

Co-authored-by: Guillaume Lessard <glessard@tffenterprises.com>

Author: stephentyrone

stdlib/public/core/UnsafePointer.swift

@@ -788,6 +788,10 @@ extension UnsafeMutablePointer where Pointee: ~Copyable {
   ///
   ///     intPointer.deallocate()
   ///
+  /// You must only use `deallocate()` to end the lifetime of memory
+  /// created with `allocate()`; it is a programming error to use `free` or
+  /// another deallocation API, and may result in undefined behavior.
+  ///
   /// - Parameter count: The amount of memory to allocate, counted in instances
   ///   of `Pointee`.
   @inlinable