stream, 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

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, stream