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 #23921)

(cherry picked from commit 6a4a714)

doc/man3/SSL_CIPHER_get_name.pod

@@ -109,7 +109,7 @@ cipher B<c>.
 
 SSL_CIPHER_description() returns a textual description of the cipher used
 into the buffer B<buf> of length B<len> provided.  If B<buf> is provided, it
-must be at least 128 bytes, otherwise a buffer will be allocated using
+must be at least 128 bytes. If B<buf> is NULL it will be allocated using
 OPENSSL_malloc().  If the provided buffer is too small, or the allocation fails,
 B<NULL> is returned.