Issue #3292980 by benjifisher, mikelutz, quietone: Testing system should explain why Guzzle responses can be unreadable

(cherry picked from commit 6b1c5e3d17)

core/tests/Drupal/Tests/BrowserHtmlDebugTrait.php

@@ -182,7 +182,9 @@ trait BrowserHtmlDebugTrait {
               /** @var \Psr\Http\Message\StreamInterface $stream */
               $stream = $response->getBody();
 
-              // Get the response body as a string.
+              // Get the response body as a string. The response stream is set
+              // to the sink, which defaults to a readable temp stream but can
+              // be overridden by setting $options['sink'].
               $body = $stream->isReadable()
                 ? (string) $stream
                 : 'Response is not readable.';