doc: clarify SSL_CIPHER_description allocation

doc: clarify SSL_CIPHER_description allocation

Previously the documentation for `SSL_CIPHER_description` said:
> If buf is provided, it must be at least 128 bytes, otherwise a buffer
> will be allocated using OPENSSL_malloc().

In reality, `OPENSSL_malloc` is only invoked if the provided `buf`
argument is `NULL`. If the `buf` arg is not `NULL`, but smaller than
128 bytes, the function returns `NULL` without attempting to allocate
a new buffer for the description.

This commit adjusts the documentation to better describe the implemented
behaviour.

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23921)