Add release note

Author: ojhunt

clang/docs/ReleaseNotes.rst

@@ -144,6 +144,11 @@ New Compiler Flags
 Deprecated Compiler Flags
 -------------------------
 
+- Use of `__has_feature` to detect the `ptrauth_qualifier` and `ptrauth_intrinsics`
+  features has been deprecated, and is restricted to the arm64e target only. The
+  correct method to check for these features is to test for the `__PTRAUTH__`
+  macro.
+
 Modified Compiler Flags
 -----------------------