Improve docs and API to clarify the purpose of `MeasureFunc`

[Weibye]

A few notes from my work on CSS grid that relates to MeasureFunc:

• For Grid, we will want to be able to pass a "sizing constraint" (MinContent or >MaxContent) as an alternative to a size (or in addition to an indefinite size).
• CSS calls the result of it's equivalent of a "MeasureFunc" an item's "intrinsic size" when sized under such constraints
• Finally, perhaps obvious to some, but MeasureFuncs only ever apply to leaf nodes and are ignored on Flex nodes. We may wish to make this clearer in the API and/or docs!

Originally posted by @nicoburns in #57 (comment)