docs: boxes: Add some more tests
docs: boxes: Add some more tests

Add shades test from bug #778122, and flesh out the diagnoals section.

https://bugzilla.gnome.org/show_bug.cgi?id=778122

thestinger/vte-ng,saitoha/vte-sixel,saitoha/vte-sixel,thestinger/vte-ng,thestinger/vte-ng,saitoha/vte-sixel