Clarify the difference between disabled and readonly

This removes the inaccurate mention that being disabled prevents
selection, which was not supported by any normative requirements and has
caused confusion for several implementers so far. Fixes #1852.

source

@@ -46950,11 +46950,22 @@ ldh-str       = &lt; as defined in <a href="https://tools.ietf.org/html/rfc1034#
 
   </div>
 
-  <p class="note">The difference between <code data-x="attr-fe-disabled">disabled</code> and <code
-  data-x="attr-input-readonly">readonly</code> is that read-only controls are still focusable, so the
-  user can still select the text and interact with it, whereas disabled controls are entirely
-  non-interactive. (For this reason, only text controls can be made read-only: it wouldn't make
-  sense for checkboxes or buttons, for instances.)</p>
+  <div class="note">
+   <p>The difference between <code data-x="attr-fe-disabled">disabled</code> and <code
+   data-x="attr-input-readonly">readonly</code> is that read-only controls can still function,
+   whereas disabled controls generally do not function as controls until they are enabled. This is
+   spelled out in more detail elsewhere in this specification with normative requirements that refer
+   to the <span data-x="concept-fe-disabled">disabled</span> concept (for example, the element's
+   <span>activation behavior</span>, whether or not it is a <span>focusable area</span>, or when
+   <span>constructing the form data set</span>). Any other behavior related to user interaction with
+   disabled controls, such as whether text can be selected or copied, is not defined in this
+   specification.</p>
+
+   <p>Only text controls can be made read-only, since for other controls (such as checkboxes and
+   buttons) there is no useful distinction between being read-only and being disabled, so the
+   <code data-x="attr-input-readonly">readonly</code> attribute <span data-x="do not apply">does not
+   apply</span>.</p>
+  </div>
 
   <div class="example">