Extensions: Update webRequest API document

Current document does not explain an 'extraHeaders' use case
to deceive the CORB inspection.

This patch updates to explain this, starting from m72.

Bug: 933893
Change-Id: I465a548de0450243d599937c20f442fa2a941017
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1878268
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Auto-Submit: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709382}

chrome/common/extensions/docs/templates/intros/webRequest.html

@@ -149,6 +149,14 @@ without specifying <code>'extraHeaders'</code> in
 </ul>
 </p>
 
+<p>
+<span class="availability">Starting from Chrome 72</span>, if you need to
+modify responses before
+<a href="https://chromium.googlesource.com/chromium/src/+/master/services/network/cross_origin_read_blocking_explainer.md">Cross Origin Read Blocking (CORB)</a>
+can block the response, you need to specify <code>'extraHeaders'</code> in
+<code>opt_extraInfpSpec</code>.
+</p>
+
 <p>
 <span class="availability">Starting from Chrome 72</span>, the following request
 headers are <strong>not provided</strong> and cannot be modified or removed