crypto: doc - add akcipher API

Reference the new akcipher API calls in the kernel crypto API DocBook.

Also, fix the comments in the akcipher.h file: double dashes do not look
good in the DocBook; fix a typo.

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