docs(treesitter): clean up and update treesitter.txt (#20142)

* add type annotations to code
* clean up and expand static documentation
* consistent use of tags for static and generated docs

scripts/gen_vimdoc.py

@@ -260,7 +260,7 @@ CONFIG = {
         'helptag_fmt': lambda name: (
             '*lua-treesitter-core*'
             if name.lower() == 'treesitter'
-            else f'*treesitter-{name.lower()}*'),
+            else f'*lua-treesitter-{name.lower()}*'),
         'fn_helptag_fmt': lambda fstem, name: (
             f'*{name}()*'
             if name != 'new'