vim: Document ctrl-s override (#30803)

Closes https://github.com/zed-industries/zed/issues/30559

Release Notes:

- N/A

docs/src/vim.md

@@ -547,6 +547,7 @@ If you're using vim mode on Linux or Windows, you may find it overrides keybindi
     "ctrl-y": "editor::Undo",          // vim default: line up
     "ctrl-f": "buffer_search::Deploy", // vim default: page down
     "ctrl-o": "workspace::Open",       // vim default: go back
+    "ctrl-s": "workspace::Save",       // vim default: show signature
     "ctrl-a": "editor::SelectAll",     // vim default: increment
   }
 },