crypto: doc - Document remaining members in struct crypto_alg

Add missing comments for union members ablkcipher, blkcipher,
cipher, and compress. This silences complaints when building
the htmldocs.

Fixes: 0d7f488f (crypto: doc - cipher data structures)
Signed-off-by: Gary R Hook <gary.hook@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>