Improce docs & examples for interfaces, modules, classes, etc.

hi, I find it very hard to move into imports, exports, declares, modules, successfully with flow, I have failed so far. And reading other issues has made it appear to me that other (even smart) people have been tripped up by all this random example: https://github.com/facebook/flow/issues/769. Thus I would like to suggest that somebody who really groks this stuff does some pair documenting with somebody who doesn't, to work up some succinct but fully fleshed out examples.