rename "grandfathered" to "min_verion"

I think this is a much more straightforward name.
add "signatures" support
add "regex_sets" support
clarify error message a bit: later versions are okay

eventually this will not be true, if experimental features are
totally expunged
make the examples consistent in use of semicolon
document the two newly-added experiments
don't show expanded forms

As the list of experiments grows, this section becomes more complex
to maintain.  It will also get more complex as experiments end,
either in success (warning removed) or failure (feature not
supported, enabling should throw an error).

We could generate this Pod later if we want, but for the most part,
I think it's much simpler to just omit this section for now.