doc: better description of KECCAK-KMAC XOF

doc: better description of KECCAK-KMAC XOF

KECCAK-KMAC-128 and KECCAK-KMAC-256 are extendable output functions
that have been defined because they are convenient for implementing
KMAC.  Give definitions for them so that users aren't left to figure
that out themselves.  KECCAK-KMAC-128 is very similar to SHAKE-128,
and KECCAK-KMAC-256 is very similar to SHAKE-256.

Related to #22619.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22755)