doc: Clarify how to generate backup files with non-exclusive backups

The current instructions describing how to write the backup_label and
tablespace_map files are confusing.  For example, opening a file in text
mode on Windows and copy-pasting the file's contents would result in a
failure at recovery because of the extra CRLF characters generated.  The
documentation was not stating that clearly, and per discussion this is
not considered as a supported scenario.

This commit extends a bit the documentation to mention that it may be
required to open the file in binary mode before writing its data.

Reported-by: Wang Shenhao
Author: David Steele
Reviewed-by: Andrew Dunstan, Magnus Hagander
Discussion: https://postgr.es/m/8373f61426074f2cb6be92e02f838389@G08CNEXMBPEKD06.g08.fujitsu.local
Backpatch-through: 9.6

Author: michaelpq

doc/src/sgml/backup.sgml

@@ -904,7 +904,8 @@ SELECT * FROM pg_stop_backup(false, true);
      <filename>backup_label</filename> in the root directory of the backup. The
      third field should be written to a file named
      <filename>tablespace_map</filename> unless the field is empty. These files are
-     vital to the backup working, and must be written without modification.
+     vital to the backup working and must be written byte for byte without 
+     modification, which may require opening the file in binary mode.
     </para>
    </listitem>
    <listitem>