Doc: Clarify the inactive_since field description.

Updated to specify that it represents the exact time a slot became
inactive, rather than the period of inactivity.

Reported-by: Peter Smith
Author: Bruce Momjian, Nisha Moond
Reviewed-by: Amit Kapila, Peter Smith
Backpatch-through: 17
Discussion: https://postgr.es/m/CAHut+PuvsyA5v8y7rYoY9mkDQzUhwaESM05yCByTMaDoRh30tA@mail.gmail.com

src/backend/replication/logical/slotsync.c

@@ -1515,7 +1515,7 @@ update_synced_slots_inactive_since(void)
 	 * correctly interpret the inactive_since if the standby gets promoted
 	 * without a restart. We don't want the slots to appear inactive for a
 	 * long time after promotion if they haven't been synchronized recently.
-	 * Whoever acquires the slot i.e.makes the slot active will reset it.
+	 * Whoever acquires the slot, i.e., makes the slot active, will reset it.
 	 */
 	if (!StandbyMode)
 		return;

src/include/replication/slot.h

@@ -205,7 +205,11 @@ typedef struct ReplicationSlot
 	 */
 	XLogRecPtr	last_saved_confirmed_flush;
 
-	/* The time since the slot has become inactive */
+	/*
+	 * The time when the slot became inactive. For synced slots on a standby
+	 * server, it represents the time when slot synchronization was most
+	 * recently stopped.
+	 */
 	TimestampTz inactive_since;
 } ReplicationSlot;