Fix authoring guidance to allow async="" on inline module scripts

Closes #3319.

Author: domenic

source

@@ -57224,13 +57224,16 @@ interface <dfn>HTMLScriptElement</dfn> : <span>HTMLElement</span> {
     a <span>classic script</span>, to be interpreted according to the JavaScript <i
     data-x="js-prod-Script">Script</i> top-level production. Classic scripts are affected by the
     <code data-x="attr-script-async">async</code> and <code data-x="attr-script-defer">defer</code>
-    attributes. Authors should omit the attribute instead of redundantly setting it.</p></li>
+    attributes, but only when the <code data-x="attr-script-src">src</code> attribute is set.
+    Authors should omit the <code data-x="attr-script-type">type</code> attribute instead of
+    redundantly setting it.</p></li>
 
     <li><p>Setting the attribute to an <span>ASCII case-insensitive</span> match for the string
     "<code data-x="">module</code>" means that the script is a <span>module script</span>, to be
     interpreted according to the JavaScript <i data-x="js-prod-Module">Module</i> top-level
     production. Module scripts are not affected by the <code data-x="attr-script-defer">defer</code>
-    attribute.</p></li>
+    attribute, but are affected by the <code data-x="attr-script-async">async</code> attribute
+    (regardless of the state of the <code data-x="attr-script-src">src</code> attribute).</p></li>
 
     <li><p>Setting the attribute to any other value means that the script is a <dfn>data
     block</dfn>, which is not processed. None of the <code>script</code> attributes (except <code
@@ -57282,11 +57285,11 @@ interface <dfn>HTMLScriptElement</dfn> : <span>HTMLElement</span> {
   data-x="attr-script-defer">defer</code></dfn> attributes are <span data-x="boolean
   attribute">boolean attributes</span> that indicate how the script should be evaluated. <span
   data-x="classic script">Classic scripts</span> may specify <code
-  data-x="attr-script-defer">defer</code> or <code data-x="attr-script-async">async</code>; <span
-  data-x="module script">module scripts</span> may specify <code
-  data-x="attr-script-async">async</code>. The <code data-x="attr-script-defer">defer</code> and
-  <code data-x="attr-script-async">async</code> attributes must not be specified if the <code
-  data-x="attr-script-src">src</code> attribute is not present.</p>
+  data-x="attr-script-defer">defer</code> or <code data-x="attr-script-async">async</code>, but must
+  not specify either unless the <code data-x="attr-script-src">src</code> attribute is present.
+  <span data-x="module script">Module scripts</span> may specify the <code
+  data-x="attr-script-async">async</code> attribute, but must not specify the <code
+  data-x="attr-script-defer">defer</code> attribute.</p>
 
   <p>There are several possible modes that can be selected using these attributes, and depending on
   the script's type.</p>