doc: two minor stream doc improvements

per: https://github.com/joyent/node/issues/14596



1. document that a runtime error will occur if you attempt
   to unshift after the end event
2. document that calling read after the end event will return
   null and will not trigger a runtime error

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