Improve low-level backup documentation.

Our documentation hasn't really caught up with the fact that
non-exclusive backups can now be taken using pg_start_backup and
pg_stop_backup even on standbys.  Update.

David Steele, reviewed by Robert Haas and Michael Paquier

Discussion: http://postgr.es/m/f349b834-1443-ebf0-3c2a-965f944004d7@pgmasters.net

doc/src/sgml/backup.sgml

@@ -889,8 +889,11 @@ SELECT pg_start_backup('label', false, false);
 <programlisting>
 SELECT * FROM pg_stop_backup(false);
 </programlisting>
-     This terminates the backup mode and performs an automatic switch to
-     the next WAL segment.  The reason for the switch is to arrange for
+     This terminates backup mode. On a primary, it also performs an automatic
+     switch to the next WAL segment.  On a standby, it is not possible to
+     automatically switch WAL segments, so you may wish to run
+     <function>pg_switch_xlog</function> on the primary to perform a manual
+     switch. The reason for the switch is to arrange for
      the last WAL segment file written during the backup interval to be
      ready to archive.
     </para>
@@ -908,7 +911,7 @@ SELECT * FROM pg_stop_backup(false);
      Once the WAL segment files active during the backup are archived, you are
      done.  The file identified by <function>pg_stop_backup</>'s first return
      value is the last segment that is required to form a complete set of
-     backup files.  If <varname>archive_mode</> is enabled,
+     backup files.  On a primary, if <varname>archive_mode</> is enabled,
      <function>pg_stop_backup</> does not return until the last segment has
      been archived.
      Archiving of these files happens automatically since you have
@@ -924,6 +927,13 @@ SELECT * FROM pg_stop_backup(false);
      <function>pg_stop_backup</> terminates because of this your backup
      may not be valid.
     </para>
+
+    <para>
+     Note that on a standby <function>pg_stop_backup</> does not wait for
+     WAL segments to be archived so the backup process must ensure that all WAL
+     segments required for the backup are successfully archived.
+    </para>
+
    </listitem>
   </orderedlist>
     </para>
@@ -932,9 +942,9 @@ SELECT * FROM pg_stop_backup(false);
     <title>Making an exclusive low level backup</title>
     <para>
      The process for an exclusive backup is mostly the same as for a
-     non-exclusive one, but it differs in a few key steps. It does not allow
-     more than one concurrent backup to run, and there can be some issues on
-     the server if it crashes during the backup. Prior to PostgreSQL 9.6, this
+     non-exclusive one, but it differs in a few key steps. This type of backup
+     can only be taken on a primary and does not allow concurrent backups.
+     Prior to <productname>PostgreSQL</> 9.6, this
      was the only low-level method available, but it is now recommended that
      all users upgrade their scripts to use non-exclusive backups if possible.
     </para>
@@ -992,6 +1002,11 @@ SELECT pg_start_backup('label', true);
      <xref linkend="backup-lowlevel-base-backup-data"> for things to
      consider during this backup.
     </para>
+    <para>
+      Note that if the server crashes during the backup it may not be
+      possible to restart until the <literal>backup_label</> file has been
+      manually deleted from the <envar>PGDATA</envar> directory.
+    </para>
    </listitem>
    <listitem>
     <para>

doc/src/sgml/func.sgml

@@ -18070,11 +18070,22 @@ postgres=# select pg_start_backup('label_goes_here');
     <function>pg_start_backup</>. In a non-exclusive backup, the contents of
     the <filename>backup_label</> and <filename>tablespace_map</> are returned
     in the result of the function, and should be written to files in the
-    backup (and not in the data directory).
+    backup (and not in the data directory).  When executed on a primary
+    <function>pg_stop_backup</> will wait for WAL to be archived, provided that
+    archiving is enabled.
    </para>
    <para>
-    The function also creates a backup history file in the transaction log
+    On a standby <function>pg_stop_backup</> will return immediately without
+    waiting, so it's important to verify that all required WAL segments have
+    been archived.  If write activity on the primary is low, it may be useful
+    to run <function>pg_switch_xlog</> on the primary in order to trigger a
+    segment switch.
+   </para>
+
+   <para>
+    When executed on a primary, the function also creates a backup history file
+    in the write-ahead log
     archive area. The history file includes the label given to
     <function>pg_start_backup</>, the starting and ending transaction log locations for
     the backup, and the starting and ending times of the backup.  The return