Correct ARM memcpy comments.

The comments on register usage in ARM memcpy had dest and src the
wrong way round; this patch (originally from Mark Shinwell) corrects
this and adds a note on the return value.

Signed-off-by: Joseph Myers <joseph@codesourcery.com>

libc/string/arm/_memcpy.S

@@ -67,8 +67,9 @@
  * a time where possible.
  *
  * Note: r12 (aka ip) can be trashed during the function along with
- * r0-r3 although r0-r2 have defined uses i.e. src, dest, len through out.
+ * r0-r3 although r0-r2 have defined uses i.e. dest, src, len throughout.
  * Additional registers are preserved prior to use i.e. r4, r5 & lr
+ * The return value in r0 must be the destination address.
  *
  * Apologies for the state of the comments ;-)
  */