[hooks alpha doc] Suggested edits for useSelector documentation

[estaub]

This is w/r/t https://github.com/reduxjs/react-redux/blob/v7-hooks-alpha/docs/api/hooks.md#useselector

I found the existing doc puzzling. If these edits are factually wrong, then please consider clarifying in that area regardless.

1. Change:

If a deps array is provided, the last saved selector will be used, and that selector will be overwritten whenever the deps array contents have changed.

to something like:

If a dependency array is provided, the selector is saved and reused until a dependency changes.

2. Change:

When an action is dispatched, useSelector() will do a shallow comparison of the previous selector result value and the current result value.

to:

If the store has been modified since useSelector() was last invoked, useSelector() does a shallow comparison of the previous and current selector result values.

Again, if the "when an action is dispatched" condition is in fact more informative in some way, please clarify.

[timdorr]

See #1267