standby clone: honour --waldir setting when cloning from Barman

By setting --waldir in "pg_basebackup_options", standbys cloned using
pg_basebackup would have their WAL directory set to the specified
location and symlinked from the data directory.

This commit causes repmgr to honour that setting even when cloning
from Barman.

doc/cloning-standbys.xml

@@ -148,6 +148,15 @@ description = "Main cluster"
       section in <command>man 5 ssh_config</command> for more details.
     </simpara>
    </tip>
+   <para>
+     If you wish to place WAL files in a location outside the main
+     PostgreSQL data directory, set <option>--waldir</option>
+     (PostgreSQL 9.6 and earlier: <option>--xlogdir</option>) in
+     <option>pg_basebackup_options</option> to the target directory
+     (must be an absolute filepath). &repmgr; will create and
+     symlink to this directory in exactly the same way
+     <application>pg_basebackup</application> would.
+   </para>
    <para>
     It's now possible to clone a standby from Barman, e.g.:
     <programlisting>
@@ -440,6 +449,13 @@ description = "Main cluster"
       WAL directory. Any WALs generated during the cloning process will be copied here, and
       a symlink will automatically be created from the main data directory.
     </para>
+    <tip>
+      <para>
+        The <literal>--waldir</literal> (<literal>--xlogdir</literal>) option,
+        if present in <varname>pg_basebackup_options</varname>, will be honoured by &repmgr;
+        when cloning from Barman (&repmgr; 5.2 and later).
+      </para>
+    </tip>
     <para>
      See the <ulink url="https://www.postgresql.org/docs/current/app-pgbasebackup.html">PostgreSQL pg_basebackup documentation</ulink>
      for more details of available options.