[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>