Add browsing session as open issue

Having a central point to acknowledge its non-existence will hopefully provide some clarity for #5560 and other changes.

source

@@ -79832,6 +79832,24 @@ interface <dfn>BarProp</dfn> {
 
   <h3 split-filename="history" id="history">Session history and navigation</h3>
 
+  <h4>Browsing sessions</h4>
+
+  <div class="XXX">
+
+  <p>A <dfn>browsing session</dfn> is …. See <a
+  href="https://github.com/whatwg/html/issues/4782">whatwg/html issue #4782</a> and <a
+  href="https://github.com/whatwg/html/issues/5350">whatwg/html issue #5350</a> for defining
+  <span>browsing session</span>. It is roughly analogous to a <span>top-level browsing
+  context</span> except that it cannot be replaced due to a `<code
+  data-x="">Cross-Origin-Opener-Policy</code>` header or navigation.</p>
+
+  <p>A <span>top-level browsing context</span> has an associated <dfn data-x="tlbc browsing session"
+  data-dfn-for="top-level browsing context">browsing session</dfn> which is a <span>browsing
+  session</span>.</p>
+
+  </div>
+
+
   <h4>The session history of browsing contexts</h4>
 
   <p>The sequence of <code>Document</code>s in a <span>browsing context</span> is its <dfn>session