The documentation still referred to only two parameters being passed to
a parameter callback, whereas the new signature takes three.

I've found myself a few times double checking what the default option is when a boolean flag is use i.e. does `default=False` refer to the first flag or the second flag.  Thought adding an example of the calls without any flags to the docs would make this a bit clearer.

use flag_value or prompt if an option is given without a value

allow specifying min and max as open separately
clamp works for open int bounds, fails for open float bounds
help output shows range information