Clarify Tag Length Setting in OCB Mode

Clarify Tag Length Setting in OCB Mode

Fixes #8331: Updated the description for setting the tag length in OCB mode to remove the misleading “when encrypting” and “during encryption” phrasing. This change emphasizes that setting a custom tag length requires a call with NULL, applicable to both encryption and decryption contexts.

Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25424)