Clarify which Feather icons aren't supported. (#15700)

Co-authored-by: Sarah Edwards <[email protected]>

Author: Kurt-von-Laven

content/actions/creating-actions/metadata-syntax-for-github-actions.md

@@ -479,11 +479,39 @@ The background color of the badge. Can be one of: `white`, `yellow`, `blue`, `gr
 
 ### `branding.icon`
 
-The name of the [Feather](https://feathericons.com/) icon to use.
+The name of the v4.28.0 [Feather](https://feathericons.com/) icon to use. Brand icons are omitted as well as the following:
+
+<table>
+<tr>
+<td>coffee</td>
+<td>columns</td>
+<td>divide-circle</td>
+<td>divide-square</td>
+</tr>
+<tr>
+<td>divide</td>
+<td>frown</td>
+<td>hexagon</td>
+<td>key</td>
+</tr>
+<tr>
+<td>meh</td>
+<td>mouse-pointer</td>
+<td>smile</td>
+<td>tool</td>
+</tr>
+<tr>
+<td>x-octagon</td>
+<td></td>
+<td></td>
+<td></td>
+</tr>
+</table>
+
+Here is an exhaustive list of all currently supported icons:
+
 <!-- 
   This table should match the icon list in `app/models/repository_actions/icons.rb` in the internal github repo.
-  This table does not match the latest version the feather library. 
-  (Brand icons are omitted, and our supported list is not necessarily up-to-date with the latest version of the feather icon library.)
   To support a new icon, update `app/models/repository_actions/icons.rb` and add the svg to `/static/images/icons/feather` in the internal github repo. 
 -->