Document that pg_basebackup will create its output directory

doc/src/sgml/ref/pg_basebackup.sgml

@@ -122,6 +122,9 @@ PostgreSQL documentation
       <listitem>
        <para>
         Directory to write the output to.
+        <application>pg_basebackup</application> will create the directory and
+        any parent directories if necessary.  The directory may already exist,
+        but it is an error if the directory already exists and is not empty.
        </para>
        <para>
         When the backup is in tar mode, and the directory is specified as