At time of writing, the AMPlay documentation only exists in plain text format.
Once I've updated the HTML version, it should be downloadable from;
http://www.mikejs.com/riscos/amplay.html
To access the plain text documentation, either use the menus within AMPlay (Menu, Help, Text from the iconbar icon), or use the Help option within the filer over the !AMPlay icon.
Alternatively, the documentation can be found inside !AMPlay, in !AMPlay.Resources.[country].Docs.Text