Since Xfce 4.10 there are no application manuals in the packages. Reason for this change is that hardly any documentation was contributed over the years mainly because people had problems with the various formats (docbook and mallard) or with the complexity of the VCS1).
To make the documentation barrier lower it was decided to start a documentation wiki where contributors, developers and translators can work on good and up-to-date documentation.
From the various dialogs and links in the Xfce applications, we redirect to the sections in this wiki.
If you want to help extending the application manuals or guides on this wiki, you can request an account on the xfce4-dev mailing list. Give yourself an introduction and we will setup an account on the wiki so you can start working!
For translators the same applies, however then send an email to the xfce-i18n mailing list. Make sure you also include the language you want to translate. Note that this only applies for an account on docs.xfce.org, information about code translation can be found here.
A number of plugins are installed to make the wiki suitable for writing documentation, at the bottom of the syntax take those plugins are listed.
Try to make screenshots that look consistent with the other previews. I (Nick) use the following setup:
Also compress the png files before uploading, with optipng for example.