pict superimpose documentation

[Hendrik Boom]

The documentation in
https://docs.racket-lang.org/pict/Pict_Combiners.html for the various
superpose combiers does not specify with argument picts cover which
other argument picts.

-- hendrik

[Joel Dueck]

The example code using the blue circle and the red rectangle makes it pretty clear. Each pict gets covered up by the one to its right in the argument list.