arch/cortex-m: Added comment for feature

Signed-off-by: Ioan-Cristian CÎRSTEA <ioan.cirstea@oxidos.io>

Author: IoanCristianCirstea

arch/cortex-m/src/lib.rs

@@ -5,6 +5,9 @@
 //! Generic support for all Cortex-M platforms.
 
 #![no_std]
+// This feature is almost stable. There is an ongoing discussion on whether the
+// method should accept non power of two values. Tock is not concerned by this
+// discussion. It also produces better code than the manual check.
 #![feature(pointer_is_aligned_to)]
 
 use core::fmt::Write;