Add notes about removing room admins

chat/element/faq.markdown

@@ -13,3 +13,16 @@ See also [Element notifications](https://readme.phys.ethz.ch/chat/element/#notif
 
 This means your messages do not use [[end-to-end encryption (e2ee)|/chat/element/e2ee]].
 Your connection to our server is still secure and [[encrypted using TLS|/chat/element/security/#encryption]].
+
+### Remove an admin from the room who left ETH
+
+This is not possible due to how the Matrix protocol works.
+Room admins power levels can only be lowered by themselves.
+An admin cannot change the power level of another admin or remove another admin.
+Please clarify and communicate such responsibilities in advance.
+See also [[/chat/element/#modifying-power-levels]].
+
+If a user left ETH and their D-PHYS Account is blocked, then their Matrix account is locked
+and they cannot effectively access the room anymore.
+However, if the user's account is later reactivated (should they return or due to collaboration),
+they would have access again. If you have security concerns we recommend then to create a new room.

chat/element/index.markdown

@@ -138,10 +138,14 @@ By default only room admins can edit these settings in **Settings** > **Roles &
 
 For example if you would like to have a moderated room, where not anyone by default can send messages, set a power level of `1` or greater in **Permissions** > **Send messages**.
 
+For important rooms we recommend to ensure that you have multiple users set as admin (level `100`). This is just in case some admin looses access to their Matrix account and to prevent loosing the ability to change room settings in that case. If there is at least one local (`@user:phys.ethz.ch`) admin in the room, then server admins (ISG) can set another local user as admin.
+
 ### Modifying power levels
 
 Any user can grant other users elevated power levels up to his own level. Any user who has a power level higher than another user can lower the other users power level. Any user can lower his own power level.
 
+Please note that these rules also mean, that an admin cannot lower the powerlevel of another admin or remove another admin from the room. And admin must do this themself before they leave ETH/D-PHYS. Therefore please clarify and communicate such responsibilities in advance.
+
 Open the user list in the right pane to view power levels. Select a username to modify the power levels:
 
 Either select a named role from the drop-down menu or set a custom power level in the highlighted box. The user shown here has been effectively muted in this room by a negative power level of `-1`. Room admins may also click on **Mute** under **Admin Tools**, which has the same effect. They may also kick or ban users. All users can ignore other users, which hides all messages from that user for you.