[openssl/openssl] f1bea8: doc: better description of KECCAK-KMAC XOF

[James Muir]

  Branch: refs/heads/master
  Home:   https://github.com/openssl/openssl
  Commit: f1bea887ef17802a2d83e9289e812c00fd0f0523
      https://github.com/openssl/openssl/commit/f1bea887ef17802a2d83e9289e812c00fd0f0523
  Author: James Muir <james at openssl.org>
  Date:   2023-11-22 (Wed, 22 Nov 2023)

  Changed paths:
    M doc/man7/EVP_MD-SHAKE.pod

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