default-config: Clarify how indexed workspace access works

resources/default-config.kdl

@@ -400,6 +400,14 @@ binds {
     Mod+Shift+U         { move-workspace-down; }
     Mod+Shift+I         { move-workspace-up; }
 
+    // You can refer to workspaces by index. However, keep in mind that
+    // niri is a dynamic workspace system, so these commands are kind of
+    // "best effort". Trying to refer to a workspace index bigger than
+    // the current workspace count will instead refer to the bottommost
+    // (empty) workspace.
+    //
+    // For example, with 2 workspaces + 1 empty, indices 3, 4, 5 and so on
+    // will all refer to the 3rd workspace.
     Mod+1 { focus-workspace 1; }
     Mod+2 { focus-workspace 2; }
     Mod+3 { focus-workspace 3; }