Add a note to explain why HTTPS Upgrading runs in main fetch

Author: meacer

fetch.bs

@@ -4527,6 +4527,10 @@ steps:
 
  <li><p>Optionally, run <a>upgrade an HTTP request</a> algorithm on <var>request</var>, if appropriate</a>.
 
+ <p class="note">HTTPS upgrading only applies to requests with <a>HTTP(S) scheme</a>s, but it's done
+ in <a>main fetch</a> instead of <a>HTTP fetch</a> to ensure that <a>upgrade a mixed content</a>
+ step runs next and applies to the upgraded request.</p>
+
  <li><p><a>Upgrade a mixed content <var>request</var> to a potentially trustworthy URL, if appropriate</a>.
 
  <li><p>If <a lt="block bad port">should <var>request</var> be blocked due to a bad port</a>,