Refine handling of node at point in treesit-indent

Refine handling of node at point in treesit-indent

* doc/lispref/modes.texi (Parser-based Indentation): Document the
semantic clearly.
* lisp/treesit.el (treesit-simple-indent-presets): Replace with
current-indentation.
(treesit--indent-1): Now NODE always starts at BOL, and parent is
always the smallest node above NODE that spans BOL.
(treesit-indent): Don't use save-excurtion, no one else does.