Fix typos in comments.

Also adjust the phrasing in the comments.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Author: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Gurjeet Singh <gurjeet@singh.im>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAPmGK17%3DPHSDZ%2B0G6jcj12buyyE1bQQc3sbp1Wxri7tODT-SDw%40mail.gmail.com
Backpatch-through: 15

src/backend/utils/activity/pgstat_database.c

@@ -421,8 +421,8 @@ pgstat_reset_database_timestamp(Oid dboid, TimestampTz ts)
 /*
  * Flush out pending stats for the entry
  *
- * If nowait is true, this function returns false if lock could not
- * immediately acquired, otherwise true is returned.
+ * If nowait is true and the lock could not be immediately acquired, returns
+ * false without flushing the entry.  Otherwise returns true.
  */
 bool
 pgstat_database_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)

src/backend/utils/activity/pgstat_function.c

@@ -186,8 +186,8 @@ pgstat_end_function_usage(PgStat_FunctionCallUsage *fcu, bool finalize)
 /*
  * Flush out pending stats for the entry
  *
- * If nowait is true, this function returns false if lock could not
- * immediately acquired, otherwise true is returned.
+ * If nowait is true and the lock could not be immediately acquired, returns
+ * false without flushing the entry.  Otherwise returns true.
  */
 bool
 pgstat_function_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)

src/backend/utils/activity/pgstat_relation.c

@@ -806,8 +806,8 @@ pgstat_twophase_postabort(TransactionId xid, uint16 info,
 /*
  * Flush out pending stats for the entry
  *
- * If nowait is true, this function returns false if lock could not
- * immediately acquired, otherwise true is returned.
+ * If nowait is true and the lock could not be immediately acquired, returns
+ * false without flushing the entry.  Otherwise returns true.
  *
  * Some of the stats are copied to the corresponding pending database stats
  * entry when successfully flushing.

src/backend/utils/activity/pgstat_subscription.c

@@ -96,8 +96,8 @@ pgstat_fetch_stat_subscription(Oid subid)
 /*
  * Flush out pending stats for the entry
  *
- * If nowait is true, this function returns false if lock could not
- * immediately acquired, otherwise true is returned.
+ * If nowait is true and the lock could not be immediately acquired, returns
+ * false without flushing the entry.  Otherwise returns true.
  */
 bool
 pgstat_subscription_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)