Clarify documentation on Caching

[ajcrites]

The new babel.config.js allows (seemingly requires) you to set up caching. I think the documentation on caching is a little unclear in a couple of spots:

Any time the using callback returns a value other than the one that was expected...

What is meant by "expected?"

Also, the .invalidate explanation seems to have been copied from the .using section but not updated. I would like to rewrite it, but I'm a bit confused about how invalidate works as well... does it invalidate the cache if it encounters a value different from the immediately previous value? Is it based on a boolean? Will it only invalidate a value that had been used with .using? In that case, how do you use them together?

Finally, the suggestion to use a boolean with .using raises a question -- this will create a cache for values of true and false. Thus it seems like once you have used both these values there is no way to invalidate the cache (without adding a .invalidate or something). I think this should either be specified if my understanding is correct or clarified if it's incorrect.

I think that the error output if you don't set up caching for babel is clearer, at least for .using:

If this function returns a value different from a previously-encountered value...