crypto: doc - clear htmldocs build warnings for crypto/hash

SPHINX build emits multiple warnings of kind:

	warning: duplicate section name 'Note'

(when building kernel via make target 'htmldocs')

This is caused by repeated use of comments of form:

	* Note: soau soaeusoa uoe

We can change the format without loss of clarity and clear the build
warnings.

Add '**[mandatory]**' or '**[optional]**' as kernel-doc field element
description prefix

This renders in HTML as (prefixes in bold)

final
    [mandatory] Retrieve result from the driver. This function finalizes the
    transformation and retrieves the resulting hash from the driver and
    pushes it back to upper layers. No data processing happens at this
    point unless hardware requires it to finish the transformation (then
    the data buffered by the device driver is processed).

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>