docs: add overlay_has_focus key context condition...

...also found in Default (Windows).sublime-keymap (build 4192).

Author: vwheeler63

docs/reference/key_bindings.md

@@ -146,6 +146,9 @@ provided by Sublime Text itself.
 : Returns `true`
   if any overlay is visible.
 
+`overlay_has_focus`
+: Returns `true` if any overlay has focus.
+
 `panel_visible`
 : Returns `true`
   if any panel is visible.