Update documentation page for translators

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/11/nls-translator.html
Description:

In the documentation for translators
(https://www.postgresql.org/docs/11/nls-translator.html) it is mentioned to
use any text editor for translating .po files. It can be very error prone
with regular text editors, especiially for non-tech people such as
translators.

I thought you it would be useful to suggest usage of dedicated editors for
.po files, such as "Free PO editor" (https://pofile.net/free-po-editor)
which will guide them how to translate, without file structure errors.