docs(emcn): trim z-index scale comment to original footprint

Correct the pre-existing scale comment in place (the old wording became
stale when DropdownMenu moved to the popover layer) rather than expanding
it. The scale tokens are global, so their documentation stays with them.

Author: waleedlatif1

apps/sim/app/_styles/globals.css

@@ -21,13 +21,8 @@
   --auth-primary-btn-hover-border: #e0e0e0;
   --auth-primary-btn-hover-text: #000000;
 
-  /* z-index scale for layered UI.
-     --z-dropdown is the in-flow panel layer that sits BELOW modals (slide-over
-     panels, floating action bars, drag overlays). Transient poppers that can be
-     opened from inside a modal — menus, selects, popovers, tooltips, toasts —
-     must sit ABOVE --z-modal so they stay visible and clickable over the modal
-     surface (the modal overlay is semi-transparent, so a popper below it is
-     faintly visible but intercepts no clicks). */
+  /* z-index scale. Transient poppers (menus, selects, popovers, tooltips, toasts)
+     sit above --z-modal so they stay clickable over the semi-transparent overlay. */
   --z-dropdown: 100;
   --z-modal: 200;
   --z-popover: 300;