improve docs on random-parameter-set selectors

[becky-gilbert]

The documentation about random-parameter-set is a little confusing, especially the part about selectors such as PERM:
https://lookit.readthedocs.io/projects/frameplayer/en/latest/randomizers/random-parameter-set.html#parameters

I think this could be improved by providing a simple example of PERM and possibly other selectors. It might also be a good idea to move some of the example text out of the parameter section, so that it just includes parameter definitions, and move examples to the bottom of the page.