doc: add a note about readable in flowing mode

Original: https://github.com/joyent/node/pull/8682



Slightly modified version of the original PR (#8682) to add
appropriate line wrapping and fix a couple of grammar nits.

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