crypto: aead - improve documentation for scatterlist layout

Properly document the scatterlist layout for AEAD ciphers.

Reported-by: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>