Update the d2i docs to reflect reality

The d2i docs state that if an error occurs then |*a| is not freed. This
is not correct. On error it is freed and set to NULL. We update the docs
to say this, and also discuss the fact that this behaviour was inconsistent
prior to OpenSSL 1.1.0.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/9146)