doc: Add link from sslinfo to pg_stat_ssl
doc: Add link from sslinfo to pg_stat_ssl

Reviewed-by: Kyotaro HORIGUCHI <4ca185ae6a495489c65959f94fb06650d122d3bf@lab.ntt.co.jp>
Discussion: ab3da337cfbbe2ed35eca0dc70b9d014d9b82261@2ndquadrant.com/

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