crypto: doc - userspace interface spec

The userspace interface of the kernel crypto API is documented with
 * a general explanation
 * a discussion of the memory in-place operation
 * the description of the message digest API
 * the description of the symmetric cipher API

The documentation refers to libkcapi as a working example on how to use
the kernel crypto API from user space.

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