If PARSER-OR-LANG is nil, it makes more sense to guess the language at
point by treesit-language-at than to simply use the first parser in
the parser list.
* doc/lispref/parsing.texi (Retrieving Nodes): Update manual.
* lisp/treesit.el (treesit-node-at)
(treesit-node-on): Guess language at point. Update docstring.
(treesit-buffer-root-node): Update docstring.
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