Correct documentation for PKCS5_PBKDF2_HMAC

Correct documentation for PKCS5_PBKDF2_HMAC

In OpenSSL 3.x, the documentation for PKCS5_PBKDF2_HMAC incorrectly states
that an iter value less than 1 is treated as a single iteration. Upon further
investigation in providers/implementations/kdfs/pbkdf2.c, it appears that
invalid iter values will result in failure and raise the
PROV_R_INVALID_ITERATION_COUNT error. This commit corrects the documentation
to accurately reflect the behavior in OpenSSL 3.x.

Closes openssl#22168

Signed-off-by: Sumitra Sharma <sumitraartsy@gmail.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22252)