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: #1041

Author: mcollina

doc/api/stream.md

@@ -595,17 +595,28 @@ possible states:
 * `readable._readableState.flowing = true`
 
 When `readable._readableState.flowing` is `null`, no mechanism for consuming the
-streams data is provided so the stream will not generate its data.
-
-Attaching a listener for the `'data'` event, calling the `readable.pipe()`
+streams data is provided so the stream will not generate its data. While in this
+state, attaching a listener for the `'data'` event, calling the `readable.pipe()`
 method, or calling the `readable.resume()` method will switch
 `readable._readableState.flowing` to `true`, causing the Readable to begin
 actively emitting events as data is generated.
 
 Calling `readable.pause()`, `readable.unpipe()`, or receiving "back pressure"
 will cause the `readable._readableState.flowing` to be set as `false`,
 temporarily halting the flowing of events but *not* halting the generation of
-data.
+data. While in this state, attaching a listener for the `'data'` event
+would not cause `readable._readableState.flowing` to switch to `true`.
+
+```js
+const { PassThrough, Writable } = require('stream');
+const pass = new PassThrough();
+const writable = new Writable();
+
+pass.pipe(writable); pass.unpipe(writable); // flowing is now false
+pass.on('data', (chunk) => { console.log(chunk.toString()); });
+pass.write('ok'); // will not emit 'data'
+pass.resume(); // must be called to make 'data' being emitted
+```
 
 While `readable._readableState.flowing` is `false`, data may be accumulating
 within the streams internal buffer.