Show the grouped role selector in the assign-a-role screenshots

The role dropdown now groups roles into General and Agent Network
tabs. Refresh the dropdown screenshot to the tabbed selector and add
a second shot of the Agent Network tab with its Admin and Usage
Viewer roles; the section text explains the grouping and when the
Agent Network tab is visible.

src/pages/manage/team/user-roles.mdx

@@ -73,9 +73,12 @@ To change a user's role, go to the `Team` tab, select the `Users` tab, and click
 <p>
   <img src="/docs-static/img/manage/team/user-tab-list.png" alt="user list in the Team tab" className="imagewrapper-big"/>
 </p>
-Select the desired role from the dropdown:
+Select the desired role from the dropdown. Roles are grouped by product surface: the `General` tab holds the account-wide roles, and the `Agent Network` tab holds the roles scoped to [Agent Network](/agent-network) (shown when Agent Network is available on the account):
 <p>
-  <img src="/docs-static/img/manage/team/user-update-role.png" alt="user role dropdown" className="imagewrapper-big"/>
+  <img src="/docs-static/img/manage/team/user-update-role.png" alt="user role dropdown with the General tab selected" className="imagewrapper-big"/>
+</p>
+<p>
+  <img src="/docs-static/img/manage/team/user-update-role-agent-network.png" alt="user role dropdown with the Agent Network tab selected" className="imagewrapper-big"/>
 </p>
 Click the `Save` button to apply the change.
 <Note>