cua-rectangle feature does offer to type ? for help. It will display several available key sequence. Since modifier is not known, a function computes the string from the prefix variable.
It is basically a switch, which works for ‘hyper or ‘super or ‘alt or ‘meta , but it does not work for ‘control.

This is feasible to use ‘control as the modifier. 
   (setq cua--rectangle-modifier-key ‘control)


This one liner patch fixes the help in such case.
The cua-rect-help.patch is the result of `diff -c` command.
The 0001- prefixed patch is git, but I believe this is not the proper format.