Stale description for pg_basebackup

Hello.

It seems to me that there's a stale description in the documentation
of pg_basebackup.

https://www.postgresql.org/docs/13/app-pgbasebackup.html

> Note that there are some limitations in taking a backup from a standby:
...
> If you are using -X none, there is no guarantee that all WAL files
> required for the backup are archived at the end of backup.

Actually, pg_basebackup waits for the all required files to be
archived, which is an established behavior by commit
52f8a59dd9@PG10. However, the same commit seems to have forgot to
change the doc for pg_basebackup. (The current description is
introduced by 9a4d51077c@PG10)

The attached is a proposal to rewrite it as the following.

+ If you are using -X none, pg_basebackup may wait for a long time for
+ all the required WAL files to be archived. In that case, You may need
+ to call pg_switch_wal() on the primary to complete it sooner.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center
diff --git a/doc/src/sgml/ref/pg_basebackup.sgml b/doc/src/sgml/ref/pg_basebackup.sgml
index a3e292d44a..73d5cbeaf7 100644
--- a/doc/src/sgml/ref/pg_basebackup.sgml
+++ b/doc/src/sgml/ref/pg_basebackup.sgml
@@ -83,8 +83,7 @@ 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.
+      If you are using <literal>-X none</literal>, pg_basebackup may wait for a long time for all the required WAL
filesto be archived.  In that case, You may need to call <function>pg_switch_wal()</function> on the primary to
completeit sooner.
 
      </para>
     </listitem>
     <listitem>