staging: lustre: libcfs: add documentation for cfs_crypto_hash_*()

Add comment blocks for cfs_crypto_hash_*() in linux-crypto.c and
libcfs_crypto.h. Delete obsolete comment about shash handling in
cfs_crypto_hash_alloc().

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5053
Reviewed-on: http://review.whamcloud.com/9990


Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>