doc: Fix obsolete description about pg_basebackup.

Previously it was documented that if using "-X none" option there was
no guarantee that all required WAL files were archived at the end of
pg_basebackup when taking a backup from the standby. But this limitation
was removed by commit 52f8a59dd9. Now, even when taking a backup
from the standby, pg_basebackup can wait for all required WAL files
to be archived. Therefore this commit removes such obsolete
description from the docs.

Also this commit adds new description about the limitation when
taking a backup from the standby, into the docs. The limitation is that
pg_basebackup cannot force the standbfy to switch to a new WAL file
at the end of backup, which may cause pg_basebackup to wait a long
time for the last required WAL file to be switched and archived,
especially when write activity on the primary is low.

Back-patch to v10 where the issue was introduced.

Reported-by: Kyotaro Horiguchi
Author: Kyotaro Horiguchi, Fujii Masao
Reviewed-by: Kyotaro Horiguchi, Fujii Masao
Discussion: https://postgr.es/m/20210420.133235.1342729068750553399.horikyota.ntt@gmail.com

doc/src/sgml/ref/pg_basebackup.sgml

@@ -85,8 +85,14 @@ PostgreSQL documentation
     </listitem>
     <listitem>
      <para>
-      If you are using <literal>-X none</literal>, there is no guarantee that all
-      WAL files required for the backup are archived at the end of backup.
+      <application>pg_basebackup</application> cannot force the standby
+      to switch to a new WAL file at the end of backup.
+      When you are using <literal>-X none</literal>, if write activity on
+      the primary is low, <application>pg_basebackup</application> may
+      need to wait a long time for the last WAL file required for the backup
+      to be switched and archived.  In this case, it may be useful to run
+      <function>pg_switch_wal</function> on the primary in order to
+      trigger an immediate WAL file switch.
      </para>
     </listitem>
     <listitem>