libpq: Send users to the wiki during grease failures

libpq: Send users to the wiki during grease failures

The existing documentation link for grease errors is fine, but something
a little more user-friendly and updatable would be ideal, since the
campaign won't last very long. The page

    https://wiki.postgresql.org/wiki/Grease

has been added for this purpose. (It's admin-protected, to avoid making
it a spam target).

Discussion: https://postgr.es/m/CAOYmi%2BkVfqgPAaR1QuY7%3DRPBYR6MsA3V3_f5%2BH-75uC06YCDDA%40mail.gmail.com