doc(ffi): clarify when to call Client::enable_automatic_backpagination

Since the boolean is read only once when subscribing the event cache,
let's make it clear when the FFI function should be called (based on
most prominent types used at the FFI layer).

bindings/matrix-sdk-ffi/src/client.rs

@@ -2069,6 +2069,9 @@ impl Client {
     ///
     /// This is an experimental feature, and might cause performance issues on
     /// large accounts. Use with caution.
+    ///
+    /// This must be called after creating a client, but before subscribing to
+    /// the event cache (so, before spawning a sync service or a timeline).
     pub fn enable_automatic_backpagination(&self) {
         self.inner.event_cache().config_mut().experimental_auto_backpagination = true;
     }