docs: document environment group version revert

applications/configure/environment-groups.mdx

@@ -161,6 +161,18 @@ porter env list
 
 This displays a table with each group's name, current version, and last updated time.
 
+## Reverting to a previous version
+
+Every update to an environment group creates a new version. If a recent change causes problems — for example, a bad value rolled out to every synced application — you can revert the group to any prior version.
+
+Reverting re-applies the target version's variables, secrets, and files as a **new** version on top of the current one. The historical version itself is preserved, so the revert is fully auditable and can itself be reverted.
+
+<Info>
+Reverting an environment group automatically redeploys every application synced to it, just like a normal update. Apps pick up the restored values on their next rollout.
+</Info>
+
+Reverts are performed through the Porter dashboard from the environment group's version history. The action is admin-scoped — only project admins can revert a group to a previous version.
+
 ## Deleting an Environment Group
 
 Environment groups can be deleted from the **Settings** tab on the environment group's page in the dashboard.