doc: stream.unshift does not reset reading state

Per https://github.com/joyent/node/issues/14604

,

Document that performing an `unshift` during a read
can have unexpected results. Following the `unshift`
with a `push('')` resets the reading state appropriately.
Also indicate that doing an `unshift` during a read
is not optimal and should be avoided.

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