[css-forms-1] Include textarea in list of elements. (#11963)

This fixes three cases where <textarea> was omitted from the list of
elements that appearance: base can be applied to.

Author: dbaron

css-forms-1/Overview.bs

@@ -80,7 +80,7 @@ Markup Shorthands: css yes, markdown yes
   To apply the [=basic appearance=] on individual controls, the following code is used:
 
   <pre class="lang-css">
-    input, meter, progress, button, select {
+    input, textarea, meter, progress, button, select {
       appearance: base;
     }
   </pre>
@@ -105,7 +105,7 @@ Markup Shorthands: css yes, markdown yes
       color: rgb(131, 17, 0);
     }
 
-    input, meter, progress, button, select {
+    input, textarea, meter, progress, button, select {
       appearance: base;
     }
     </pre>
@@ -122,7 +122,7 @@ Markup Shorthands: css yes, markdown yes
       color: rgb(0, 249, 0);
     }
 
-    input, meter, progress, button, select {
+    input, textarea, meter, progress, button, select {
       appearance: base;
     }
     </pre>