DOC: Add note on how to terminate an OSSL_PARAM array

DOC: Add note on how to terminate an OSSL_PARAM array

The examples are also updated to have correct terminators.

doc/man3/OSSL_PARAM.pod is deliberately written with no help from the
constructor macros described in OSSL_PARAM_int.pod.  Therefore, use of
OSSL_PARAM_END isn't shown directly here, only leaving a link to its
man-page to indicate that there is that option.

Fixes #11280

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/13478)