Make Parser + PendingReadError final, add explicit note about extension to Message

lib/Message.php

@@ -11,6 +11,9 @@ use Amp\Success;
  * Creates a buffered message from an InputStream. The message can be consumed in chunks using the read() API or it may
  * be buffered and accessed in its entirety by waiting for the promise to resolve.
  *
+ * Other implementations may extend this class to add custom properties such as a `isBinary()` flag for WebSocket
+ * messages.
+ *
  * Buffering Example:
  *
  * $stream = new Message($inputStream);
@@ -84,6 +87,7 @@ class Message implements InputStream, Promise {
         return $this->buffer;
     }
 
+    /** @inheritdoc */
     final public function read(): Promise {
         if ($this->pendingRead) {
             throw new PendingReadError;
@@ -114,9 +118,7 @@ class Message implements InputStream, Promise {
         return $this->pendingRead->promise();
     }
 
-    /**
-     * {@inheritdoc}
-     */
+    /** @inheritdoc */
     final public function onResolve(callable $onResolved) {
         $this->buffering = true;
 

lib/Parser.php

@@ -7,7 +7,7 @@ use Amp\InvalidYieldError;
 use Amp\Promise;
 use Amp\Success;
 
-class Parser implements OutputStream {
+final class Parser implements OutputStream {
     /** @var \Generator */
     private $generator;
 

lib/PendingReadError.php

@@ -5,7 +5,7 @@ namespace Amp\ByteStream;
 /**
  * Thrown in case a second read operation is attempted while another read operation is still pending.
  */
-class PendingReadError extends \Error {
+final class PendingReadError extends \Error {
     public function __construct(
         string $message = "The previous read operation must complete before read can be called again",
         int $code = 0,