Improve documentation on supply/react/whenever

[JJ]

Problem or new feature

This documentation needs a series of improvements. Grammar is awkward sometimes, the examples should try and use real uses cases (#1748 ), but there is also an error in the last example, that mentions "a second argument can be supplied to interval", but does not use it in the example.

Suggestions

Check out the documentation, and also provide output examples; there's no output anywhere to be seen.
If applicable, suggest something towards the solution.