[BYTEORDER]: Document alignment and byteorder macros

This patch comments the fact that although passing le64_to_cpup et
al. is within the intended use of the byteorder macros, using
get_unaligned is the recommended way to go.
Signed-off-by: Ed L. Cashin <ecashin@coraid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/byteorder/generic.h

@@ -5,6 +5,10 @@
  * linux/byteorder_generic.h
  * Generic Byte-reordering support
  *
+ * The "... p" macros, like le64_to_cpup, can be used with pointers
+ * to unaligned data, but there will be a performance penalty on 
+ * some architectures.  Use get_unaligned for unaligned data.
+ *
  * Francois-Rene Rideau <fare@tunes.org> 19970707
  *    gathered all the good ideas from all asm-foo/byteorder.h into one file,
  *    cleaned them up.