DOC: config: Fix pgsql-check documentation to make user param mandatory

The username is required in the Start-up message. Thus, since the 2.2, when
this health-check was refactored, the user parameter is mandatory. On prior
versions, when no username is provided, no pgsql check is performed but only
a basic tcpcheck.

This patch should be backported as far as 2.2.
1 file changed