add ClearScrollback example that adds CTRL-L after clearing

refs: https://github.com/wez/wezterm/issues/2290

docs/config/lua/keyassignment/ClearScrollback.md

@@ -14,6 +14,8 @@ local act = wezterm.action
 return {
   keys = {
     -- Clears only the scrollback and leaves the viewport intact.
+    -- You won't see a difference in what is on screen, you just won't
+    -- be able to scroll back until you've output more stuff on screen.
     -- This is the default behavior.
     {
       key = 'K',
@@ -26,6 +28,16 @@ return {
       mods = 'CTRL|SHIFT',
       action = act.ClearScrollback 'ScrollbackAndViewport',
     },
+    -- Clears the scrollback and viewport, and then sends CTRL-L to ask the
+    -- shell to redraw its prompt
+    {
+      key = 'K',
+      mods = 'CTRL|SHIFT',
+      action = act.Multiple {
+        act.ClearScrollback 'ScrollbackAndViewport',
+        act.SendKey { key = 'L', mods = 'CTRL' },
+      },
+    },
   },
 }
 ```