doc: Add a note on PO editors

While PO files can be edited in any text editor, specialized tools for
translation editing can be quite helpful with automating tasks etc. Add
a small note about PO editors to encourage new translators to research
which tool works best for them.

Reviewed-by: Bruce Momjian <bruce@momjian.us>
Discussion: https://postgr.es/m/163490116698.684.10398197970578456928@wrigleys.postgresql.org

doc/src/sgml/nls.sgml

@@ -203,7 +203,9 @@ make update-po
    <title>Editing the PO Files</title>
 
    <para>
-    The PO files can be edited with a regular text editor.  The
+    The PO files can be edited with a regular text editor. There are also
+    several specialized editors for PO files which can help the process with
+    translation specific features.  The
     translator should only change the area between the quotes after
     the msgstr directive, add comments, and alter the fuzzy flag.
     There is (unsurprisingly) a PO mode for Emacs, which I find quite