Doc for createReaction should be more detailed

[viridia]

The documentation for createRoot() is really sparse - just two sentences, one which describes the motivation, and the other which describes the functionality. That's not really enough.

In particular, I would like it to be more explicit about the owner scope in which the tracked function runs. I had initially assumed that the tracked function inherited the scope that was current when createReaction() is called, but that is not the case. Instead, the tracked function inherits the scope that is current when the function returned from createReaction is called.