docs(ui-conversation): state the back-to-bottom offset as geometry

The composer and back-to-bottom stacking comments now state the resting
16px gap instead of claiming the control never overlaps the input card.

Refs #7590

packages/client/ui-chat/src/client/chat/ChatView.module.css

@@ -347,7 +347,7 @@
   /* Above transcript sticky chrome (CodeBlock banners 6, the pinned compaction
      header and turn rail 7) and below the composer seat (9, ui-conversation
      ConversationRoot), so menus the composer opens upward cover this control.
-     The control rests above the seat, so it does not overlap the input card. */
+     At rest the control sits 16px above the input card. */
   z-index: 8;
   height: 0;
   display: flex;

packages/client/ui-conversation/src/client/skeleton/ConversationRoot.module.css

@@ -503,7 +503,7 @@
      the pinned compaction header (7, MessageItem.module.css) and turn rail (7,
      TurnNavigator.module.css) in ui-chat, and the back-to-bottom control (8,
      ChatView.module.css), so menus the composer opens upward cover that
-     control. The control sits above the seat and never overlaps the card. */
+     control. At rest the control sits 16px above the input card. */
   z-index: 9;
   /* A direct compositing reason: without it the seat is composited only while
      it overlaps other composited content, so its layer could come and go with