doc: Clarify meaning of "idle" in idle_replication_slot_timeout.

doc: Clarify meaning of "idle" in idle_replication_slot_timeout.

This commit updates the documentation to clarify that "idle" in
idle_replication_slot_timeout means the replication slot is inactive,
that is, not currently used by any replication connection.

Without this clarification, "idle" could be misinterpreted to mean
that the slot is not advancing or that no data is being streamed,
even if a connection exists.

Back-patch to v18 where idle_replication_slot_timeout was added.

Author: Laurenz Albe <[email protected]>
Reviewed-by: David G. Johnston <[email protected]>
Reviewed-by: Gunnar Morling <[email protected]>
Reviewed-by: Amit Kapila <[email protected]>
Reviewed-by: Fujii Masao <[email protected]>
Discussion: https://postgr.es/m/CADGJaX_0+FTguWpNSpgVWYQP_7MhoO0D8=cp4XozSQgaZ40Odw@mail.gmail.com
Backpatch-through: 18