Improve docs for timing and skipping of checkpoints

Greg Smith

doc/src/sgml/wal.sgml

@@ -424,8 +424,15 @@
    linkend="guc-checkpoint-segments"> log segments, or every <xref
    linkend="guc-checkpoint-timeout"> seconds, whichever comes first.
    The default settings are 3 segments and 300 seconds (5 minutes), respectively.
-   It is also possible to force a checkpoint by using the SQL command
-   <command>CHECKPOINT</command>.
+   In cases where there are little or no writes to the WAL, checkpoints will be
+   skipped even if checkpoint_timeout has passed.  At least one new WAL segment
+   must have been created before an automatic checkpoint occurs.  The time
+   between checkpoints and when new WAL segments are created are not related
+   in any other way.  If file-based WAL shipping is being used and you want to
+   bound how often files are sent to standby server, to reduce potential data
+   loss you should adjust archive_timeout parameter rather than the checkpoint
+   parameters.  It is also possible to force a checkpoint by using the SQL
+   command <command>CHECKPOINT</command>.
   </para>
 
   <para>