Clarify logout key backup warning dialog (#8741)

Added details to the logout key backup warning dialog on
why access to encrypted messages will be lost.

Fix https://github.com/vector-im/element-web/issues/15565

Co-authored-by: notramo <notramo@protonmail.com>

src/components/views/dialogs/LogoutDialog.tsx

@@ -134,6 +134,12 @@ export default class LogoutDialog extends React.Component<IProps, IState> {
                     "Encrypted messages are secured with end-to-end encryption. " +
                     "Only you and the recipient(s) have the keys to read these messages.",
                 ) }</p>
+                <p>{ _t(
+                    "When you sign out, these keys will be deleted from this device, " +
+                    "which means you won't be able to read encrypted messages unless you " +
+                    "have the keys for them on your other devices, or backed them up to the " +
+                    "server.",
+                ) }</p>
                 <p>{ _t("Back up your keys before signing out to avoid losing them.") }</p>
             </div>;