Add docs for tree-sitter textobjects

book/src/usage.md

@@ -49,9 +49,10 @@ Multiple characters are currently not supported, but planned.
 
 ## Textobjects
 
-Currently supported: `word`, `surround`.
+Currently supported: `word`, `surround`, `function`, `class`.
 
 ![textobject-demo](https://user-images.githubusercontent.com/23398472/124231131-81a4bb00-db2d-11eb-9d10-8e577ca7b177.gif)
+![textobject-treesitter-demo](https://user-images.githubusercontent.com/23398472/132537398-2a2e0a54-582b-44ab-a77f-eb818942203d.gif)
 
 - `ma` - Select around the object (`va` in vim, `<alt-a>` in kakoune)
 - `mi` - Select inside the object (`vi` in vim, `<alt-i>` in kakoune)
@@ -60,5 +61,10 @@ Currently supported: `word`, `surround`.
 | ---                    | ---                      |
 | `w`                    | Word                     |
 | `(`, `[`, `'`, etc     | Specified surround pairs |
+| `f`                    | Function                 |
+| `c`                    | Class                    |
 
-Textobjects based on treesitter, like `function`, `class`, etc are planned.
+Note: `f`, `c`, etc need a tree-sitter grammar active for the current
+document and a special tree-sitter query file to work properly. [Only
+some grammars](https://github.com/search?q=repo%3Ahelix-editor%2Fhelix+filename%3Atextobjects.scm&type=Code&ref=advsearch&l=&l=)
+currently have the query file implemented. Contributions are welcome !