d2i_X509.pod: Better document using the reuse capability for libctx setup

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22265)

(cherry picked from commit 4ca56f51)