Point out how media.buffered/played/seekable are broken

Hopefully this lessens the likelihood of the pattern being copied.

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=22471.

source

@@ -33686,6 +33686,9 @@ interface <dfn>HTMLMediaElement</dfn> : <span>HTMLElement</span> {
   data-x="dom-media-buffered">buffered</code> attribute at one time can end up being not included in
   the range(s) of objects returned by the same attribute at later times.</p>
 
+  <p class="warning">Returning a new object each time is a bad pattern for attribute getters and is
+  only enshrined here as it would be costly to change it. It is not to be copied to new APIs.</p>
+
   </div>
 
 
@@ -34555,6 +34558,9 @@ interface <dfn>HTMLMediaElement</dfn> : <span>HTMLElement</span> {
   increase of the <span>current playback position</span> during normal playback, if any, at the time
   the attribute is evaluated.</p>
 
+  <p class="warning">Returning a new object each time is a bad pattern for attribute getters and is
+  only enshrined here as it would be costly to change it. It is not to be copied to new APIs.</p>
+
   <hr>
 
   <p>Each <span>media element</span> has a <dfn>list of pending play promises</dfn>, which must
@@ -35208,6 +35214,9 @@ interface <dfn>HTMLMediaElement</dfn> : <span>HTMLElement</span> {
   sliding window on an infinite stream. This is the behavior seen with DVRs viewing live TV, for
   instance.</p>
 
+  <p class="warning">Returning a new object each time is a bad pattern for attribute getters and is
+  only enshrined here as it would be costly to change it. It is not to be copied to new APIs.</p>
+
   <p>User agents should adopt a very liberal and optimistic view of what is seekable. User
   agents should also buffer recent content where possible to enable seeking to be fast.</p>