lib: memzero_explicit: add comment for its usage

Lets improve the comment to add a note on when to use memzero_explicit()
for those not digging through the git logs. We don't want people to
pollute places with memzero_explicit() where it's not really necessary.

Reference: https://lkml.org/lkml/2015/1/4/190
Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

lib/string.c

@@ -604,6 +604,11 @@ EXPORT_SYMBOL(memset);
  * @s: Pointer to the start of the area.
  * @count: The size of the area.
  *
+ * Note: usually using memset() is just fine (!), but in cases
+ * where clearing out _local_ data at the end of a scope is
+ * necessary, memzero_explicit() should be used instead in
+ * order to prevent the compiler from optimising away zeroing.
+ *
  * memzero_explicit() doesn't need an arch-specific version as
  * it just invokes the one of memset() implicitly.
  */