doc: resume a stream after pipe() and unpipe()

Clarifies the behavior of streams when _readableState.flowing is
false. resume() must be called explicitly for the 'data' event to
be emitted again.

Fixes: https://github.com/nodejs/node/issues/1041
PR-URL: https://github.com/nodejs/node/pull/13329


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>