Clarify EVP_CipherUpdate() authenticated bytes behavior

Fixes #8310: Document that the number of authenticated bytes returned by EVP_CipherUpdate() varies with the cipher used. Mention that stream ciphers like ChaCha20 can handle 1 byte at a time, while OCB mode requires processing data one block at a time. Ensure it's clear that passing unpadded data in one call is safe.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24961)

(cherry picked from commit d15077d3)