Fix treesit-node-field-name and friends (bug#66674)

Fix treesit-node-field-name and friends (bug#66674)

So turns out ts_node_field_name_for_child takes a named node index,
but we were passing it normal index that counts both named and
anonymous nodes.  That's what makes the field name all wrong in
treesit explorer.

* doc/lispref/parsing.texi:
(Accessing Node Information): Update docstring.
* lisp/treesit.el (treesit-node-index): Add some unrelated comment.
(treesit-node-field-name): Get named node index rather than all node
index.
* src/treesit.c (Ftreesit_node_field_name_for_child): Update
docstring, use ts_node_named_child_count.