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 <laurenz.albe@cybertec.at>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Gunnar Morling <gunnar.morling@googlemail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CADGJaX_0+FTguWpNSpgVWYQP_7MhoO0D8=cp4XozSQgaZ40Odw@mail.gmail.com
Backpatch-through: 18

doc/src/sgml/config.sgml

@@ -4618,10 +4618,12 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
       </term>
       <listitem>
        <para>
-        Invalidate replication slots that have remained idle longer than this
-        duration. If this value is specified without units, it is taken as
-        seconds. A value of zero (the default) disables the idle timeout
-        invalidation mechanism. This parameter can only be set in the
+        Invalidate replication slots that have remained inactive (not used by
+        a <link linkend="protocol-replication">replication connection</link>)
+        for longer than this duration.
+        If this value is specified without units, it is taken as seconds.
+        A value of zero (the default) disables the idle timeout
+        invalidation mechanism.  This parameter can only be set in the
         <filename>postgresql.conf</filename> file or on the server command
         line.
        </para>

doc/src/sgml/system-views.sgml

@@ -2929,7 +2929,7 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
         <listitem>
          <para>
           <literal>idle_timeout</literal> means that the slot has remained
-          idle longer than the configured
+          inactive longer than the configured
           <xref linkend="guc-idle-replication-slot-timeout"/> duration.
          </para>
         </listitem>

src/backend/replication/slot.c

@@ -1892,7 +1892,7 @@ InvalidatePossiblyObsoleteSlot(uint32 possible_causes,
 		 * max_slot_wal_keep_size is set to -1 and
 		 * idle_replication_slot_timeout is set to 0 during the binary
 		 * upgrade. See check_old_cluster_for_valid_slots() where we ensure
-		 * that no invalidated before the upgrade.
+		 * that no slot was invalidated before the upgrade.
 		 */
 		Assert(!(*invalidated && SlotIsLogical(s) && IsBinaryUpgrade));