Overhaul of the text viewer chapter in the manual - the old one still described the...

Overhaul of the text viewer chapter in the manual - the old one still described the not-existent-anymore button combos to change specific view modes etc. and which was replaced by a menu for quite a while already. Hence update the button table and describe the menu. Also mention the setting save (there was outdated info)  and bookmarking (not mentioned yet). Wording could probably improved but at least the information is up-to-date...

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@18581 a1c6a512-1295-4272-9138-f99709370657