Improve documentation about max_replication_slots

In the PostgreSQL connector documentation, the example in Step 2 of the topic Configuring the PostgreSQL server shows the max_replication_slots and max_wal_senders settings set to the value 1.
Users might be misled into thinking that these are realistic settings for these configuration properties. In PostgreSQL 10+ the default is actually 10 and if the value configured is less than the number of existing replication slots, PostgreSQL will fail to start.

We probably should either a) remove this setting from the documentation or b) update the documentation to be more clear about what value this should really be set to and why. This question was raised via an email from Andrey Ignatenko about usage of the PG connector.