doc/man3/X509_LOOKUP_meth_new.pod: clarify the requirements

The documentation of what a X509_LOOKUP implementation must do was
unclear and confusing.  Most of all, clarification was needed that it
must store away the found objects in the X509_STORE.

Fixes #8707

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8755)