[docs] Document terminal scrollbar command-history markers

src/frontend/src/content/docs/dashboard/terminals.mdx

@@ -201,6 +201,12 @@ A detached window starts with the originating viewer's selected font size, then
 
 Multiple viewers can share a terminal, but only one is the **primary** viewer that controls the process's grid dimensions. Opening a detached window requests that role. Revealing or returning to an active dock pane also requests sizing control. Explicit sizing actions can request control; ordinary typing and pasting don't. A viewer that loses control doesn't continuously reclaim it from another viewer.
 
+## Browse command history with the scrollbar
+
+The terminal's overlay scrollbar shows circular markers for OSC 133 command checkpoints alongside the scroll thumb, so you can jump to earlier commands without scrolling through their output line by line. When markers would overlap the thumb, the renderer displaces the nearby markers so they stay visible and clickable beside it. The track and thumb colors follow the active terminal palette, and the markers keep their own color so they remain visible against either palette.
+
+Command marks remain available for as long as their backing scrollback text is retained, rather than being evicted after a fixed number of commands. Very old marks can still be dropped once their backing text scrolls out of the retained history.
+
 ## Navigate with the keyboard
 
 Terminal applications handle many keys themselves, including keys normally used to navigate a web page.