crypto: doc - cipher data structures

The data structure of struct crypto_alg together with various other
data structures needed by cipher developers is documented wit all
parameters that can be set by a developer of a transformation. All
parameters that are internal to the crypto API are marked as such.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>