Placeholder text for <iframe> encourages inaccessible content

Iframe unit in the studio have accesibility issue as there was no title for
Iframe provided as well as placeholder does not specify what changes to made
to add new iframe

TNL-1073

common/lib/xmodule/xmodule/templates/html/iframe.yaml

@@ -8,5 +8,7 @@ data: |
       <p>To add the exercise or tool in an IFrame, replace the URL in the example tool below with the URL of the page that contains the exercise or tool that you want. Note that the URL must start with https instead of http so that the IFrame will appear in all browsers.</p>
       <p>For more information about IFrames and their attributes, <a href="http://www.w3.org/wiki/HTML/Elements/iframe">see the IFrame specification.</a></p>
       <p>In the image below, drag any of the red points to a new location. As you drag the point, notice the way the point's position affects the triangle's Euler line.</p>
-      <iframe src="https://studio.edx.org/c4x/edX/DemoX/asset/eulerLineDemo.html" width="402" height="402" marginwidth="0" marginheight="0" frameborder="0" scrolling="no">You need an iFrame capable browser to view this.</iframe>
+      <iframe placeholder="To add the exercise or tool in an IFrame, replace the URL in the example tool below with the URL of the page that contains the exercise or tool that you want. Note that the URL must start with https instead of http so that the IFrame will appear in all browsers. You must also replace the value of the title attribute with text that describes the external content. This text is not visible, but can be accessed by screen reader users." title="Euler Line Demo" src="https://studio.edx.org/c4x/edX/DemoX/asset/eulerLineDemo.html" width="402" height="402" marginwidth="0" marginheight="0" frameborder="0" scrolling="no">
+      To add the exercise or tool in an IFrame, replace the URL in the example tool below with the URL of the page that contains the exercise or tool that you want. Note that the URL must start with https instead of http so that the IFrame will appear in all browsers. You must also replace the value of the title attribute with text that describes the external content. This text is not visible, but can be accessed by screen reader users.
+      </iframe>