Linkify mentions of the primary/subscriber's max_replication_slots

In the replication server configuration documentation[0], there are
3 sections: sending, primary, standby, and subscriber. These sections
mention various GUCs and how they apply to each server configuration.

The max_replication_slots GUC is mentioned twice, once in the sending
section[1]:

> [...]
>
> Note that this parameter also applies on the subscriber side, but with
> a different meaning.

and another time in the subscriber section[2]:

> [...]
>
> Note that this parameter also applies on a sending server, but with
> a different meaning.

We can aid users that read these sections of the docs by adding links to
the max_replication_slots GUC description which the texts refer to.

Here is a patch which does so.

[0]: https://www.postgresql.org/docs/17/runtime-config-replication.html
[1]: https://www.postgresql.org/docs/17/runtime-config-replication.html#GUC-MAX-REPLICATION-SLOTS
[2]: https://www.postgresql.org/docs/17/runtime-config-replication.html#GUC-MAX-REPLICATION-SLOTS-SUBSCRIBER

--
Tristan Partin
Neon (https://neon.tech)