Updates documentation of emitters

- Ensures they do not discuss output buffering.
- Adds section on `SapiStreamEmitter`.

doc/book/emitting-responses.md

@@ -3,10 +3,15 @@
 If you are using a non-SAPI PHP implementation and wish to use the `Server` class, or if you do not
 want to use the `Server` implementation but want to emit a response, this package provides an
 interface, `Zend\Diactoros\Response\EmitterInterface`, defining a method `emit()` for emitting the
-response. A single implementation is currently available, `Zend\Diactoros\Response\SapiEmitter`,
-which will use the native PHP functions `header()` and `echo` in order to emit the response. If you
-are using a non-SAPI implementation, you will need to create your own `EmitterInterface`
-implementation.
+response.
+
+Diactoros provides two implementations currently, both for working with
+traditional Server API (SAPI) implementations: `Zend\Diactoros\Response\SapiEmitter`
+and `Zend\Diactoros\Response\SapiStreamEmitter`.  Each uses the native `header()`
+PHP function to emit headers, and `echo()` to emit the response body.
+
+If you are using a non-SAPI implementation, you will need to create your own
+`EmitterInterface` implementation.
 
 For example, the `SapiEmitter` implementation of the `EmitterInterface` can be used thus:
 
@@ -16,3 +21,43 @@ $response->getBody()->write("some content\n");
 $emitter = new Zend\Diactoros\Response\SapiEmitter();
 $emitter->emit($response);
 ```
+
+## Emitting ranges of streamed files
+
+The `SapiStreamEmitter` is useful when you want to emit a `Content-Range`. As an
+example, to stream a range of bytes from a file to a client, the client can pass
+the following header:
+
+```http
+Range: bytes=1024-2047
+```
+
+Your application would then populate the response with a `Content-Range` header:
+
+```php
+$range = $request->getHeaderLine('range');
+$range = str_replace('=', ' ', $range);
+
+$body = new Stream($pathToFile);
+$size = $body->getSize();
+$range .= '/' . $size;
+
+$response = new Response($body);
+$response = $response->withHeader('Content-Range', $range);
+```
+
+> Note: you will likely want to ensure the range specified falls within the
+> content size of the streamed body!
+
+The `SapiStreamEmitter` detects the `Content-Range` header and emits only the
+bytes specified.
+
+```php
+$emitter = new SapiStreamEmitter();
+$emitter->emit($response);
+```
+
+The `SapiStreamEmitter` may be used in place of the `SapiEmitter`, even when not
+sending files. However, unlike the `SapiEmitter`, it will emit a chunk of
+content at a time instead of the full content at once, which could lead to
+performance overhead. The default chunk size is 8192 bytes.