crypto: doc - AEAD / RNG AF_ALG interface

The patch moves the information provided in
Documentation/crypto/crypto-API-userspace.txt into a separate chapter in
the kernel crypto API DocBook. Some corrections are applied (such as
removing a reference to Netlink when the AF_ALG socket is referred to).

In addition, the AEAD and RNG interface description is now added.

Also, a brief description of the zero-copy interface with an example
code snippet is provided.

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