crypto: doc - clarify hash callbacks state machine

Add a note that it is perfectly legal to "abandon" a request object:
- call .init() and then (as many times) .update()
- _not_ call any of .final(), .finup() or .export() at any point in
  future

Link: https://lkml.kernel.org/r/20180222114741.GA27631@gondor.apana.org.au


Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>