doc: readable event clarification

per https://github.com/joyent/node/issues/14597



Indicate that `'readable'` indicates only that data can
be read from the stream, not that there is actually data
to be consumed. `readable.read([size])` can still return
null. Includes an example that illustrates the point.

Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/joyent/node/pull/25591