Document new setting to require secure backup

This adds notes on configuring the new `.well-known` setting to require Element
users to set up secure backup before continuing into the app.

Part of #14954

docs/e2ee.md

@@ -7,6 +7,7 @@ For private room creation, Element will default to encryption on but give you th
 
 Set the following on your homeserver's
 `/.well-known/matrix/client` config:
+
 ```json
 {
   "im.vector.e2ee": {
@@ -15,6 +16,25 @@ Set the following on your homeserver's
 }
 ```
 
+# Secure backup
+
+By default, Element strongly encourages (but does not require) users to set up
+Secure Backup so that cross-signing identity key and message keys can be
+recovered in case of a disaster where you lose access to all active devices.
+
+## Requiring secure backup
+
+To require Secure Backup to be configured before Element can be used, set the
+following on your homeserver's `/.well-known/matrix/client` config:
+
+```json
+{
+  "im.vector.e2ee": {
+    "secureBackupRequired": true
+  }
+}
+```
+
 # Compatibility
 
 The settings above were first proposed under a `im.vector.riot.e2ee` key, which