doc/man3/OPENSSL_malloc.pod: document OPENSSL_aligned_alloc peculiarity

doc/man3/OPENSSL_malloc.pod: document OPENSSL_aligned_alloc peculiarity

OPENSSL_aligned_alloc can return NULL in cases other than memory
exhaustion or incorrect arguments, document that.

Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28059)