Fix documentation regarding KMAC sizes

Fix documentation regarding KMAC sizes

As per recommendation by jfinkhaeuser, this documents the defaults for
KMAC-128 as 32 and for KMAC-256 as 64. The code already accomodates for
these values, so no changes are needed there.

Fixes #22381

CLA: trivial

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22614)