Make treesit-node-at/on guess language at point

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.

doc/lispref/parsing.texi

@@ -576,12 +576,12 @@ leaf node after @var{pos}.
 Finally, if there is no leaf node after @var{pos}, return the first
 leaf node before @var{pos}.
 
-When @var{parser-or-lang} is @code{nil} or omitted, this function uses
+If @var{parser-or-lang} is a parser object, this function uses that
-the first parser in @code{(treesit-parser-list)} of the current
+parser; if @var{parser-or-lang} is a language, this function uses the
-buffer.  If @var{parser-or-lang} is a parser object, it uses that
+first parser for that language in the current buffer, or creates one
-parser; if @var{parser-or-lang} is a language, it finds the first
+if none exists; if @var{parser-or-lang} is @code{nil}, this function
-parser using that language in @code{(treesit-parser-list)}, and uses
+tries to guess the language at @var{pos} by
-that.
+@code{treesit-language-at}.
 
 If this function cannot find a suitable node to return, it returns
 @code{nil}.
@@ -610,13 +610,14 @@ is at or after @var{end}.
 inside any top-level construct (function definition, etc.) most
 probably will give you the root node, because the root node is the
 smallest node that covers that empty line.  Most of the time, you want
-to use @code{treesit-node-at}, described above, instead.
+to use @code{treesit-node-at} instead.
 
-When @var{parser-or-lang} is @code{nil}, this function uses the first
+If @var{parser-or-lang} is a parser object, this function uses that
-parser in @code{(treesit-parser-list)} of the current buffer.  If
+parser; if @var{parser-or-lang} is a language, this function uses the
-@var{parser-or-lang} is a parser object, it uses that parser; if
+first parser for that language in the current buffer, or creates one
-@var{parser-or-lang} is a language, it finds the first parser using
+if none exists; if @var{parser-or-lang} is @code{nil}, this function
-that language in @code{(treesit-parser-list)}, and uses that.
+tries to guess the language at @var{beg} by
+@code{treesit-language-at}.
 
 If @var{named} is non-@code{nil}, this function looks for a named node
 only (@pxref{tree-sitter named node, named node}).
@@ -628,9 +629,10 @@ This function returns the root node of the syntax tree generated by
 @end defun
 
 @defun treesit-buffer-root-node &optional language
-This function finds the first parser that uses @var{language} in
+This function finds the first parser for @var{language} in the current
-@code{(treesit-parser-list)} of the current buffer, and returns the
+buffer, or creates one if none exists, and returns the root node
-root node generated by that parser.  If it cannot find an appropriate
+generated by that parser.  If @var{language} is omitted, it uses the
+first parser in the parser list.  If it cannot find an appropriate
 parser, it returns @code{nil}.
 @end defun
 

lisp/treesit.el

@@ -171,13 +171,15 @@ before POS.
 Return nil if no leaf node can be returned.  If NAMED is non-nil,
 only look for named nodes.
 
-If PARSER-OR-LANG is nil, use the first parser in
+If PARSER-OR-LANG is a parser, use that parser; if PARSER-OR-LANG
-`treesit-parser-list'; if PARSER-OR-LANG is a parser, use
+is a language, find the first parser for that language in the
-that parser; if PARSER-OR-LANG is a language, find a parser using
+current buffer, or create one if none exists; If PARSER-OR-LANG
-that language in the current buffer, and use that."
+is nil, try to guess the language at POS by
+`treesit-language-at'."
   (let* ((root (if (treesit-parser-p parser-or-lang)
                    (treesit-parser-root-node parser-or-lang)
-                 (treesit-buffer-root-node parser-or-lang)))
+                 (treesit-buffer-root-node
+                  (or parser-or-lang (treesit-language-at pos)))))
          (node root)
          (node-before root)
          (pos-1 (max (1- pos) (point-min)))
@@ -219,13 +221,15 @@ to use `treesit-node-at' instead.
 Return nil if none was found.  If NAMED is non-nil, only look for
 named node.
 
-If PARSER-OR-LANG is nil, use the first parser in
+If PARSER-OR-LANG is a parser, use that parser; if PARSER-OR-LANG
-`treesit-parser-list'; if PARSER-OR-LANG is a parser, use
+is a language, find the first parser for that language in the
-that parser; if PARSER-OR-LANG is a language, find a parser using
+current buffer, or create one if none exists; If PARSER-OR-LANG
-that language in the current buffer, and use that."
+is nil, try to guess the language at BEG by
+`treesit-language-at'."
   (let ((root (if (treesit-parser-p parser-or-lang)
                   (treesit-parser-root-node parser-or-lang)
-                (treesit-buffer-root-node parser-or-lang))))
+                (treesit-buffer-root-node
+                 (or parser-or-lang (treesit-language-at beg))))))
     (treesit-node-descendant-for-range root beg (or end beg) named)))
 
 (defun treesit-node-top-level (node &optional type)
@@ -246,10 +250,10 @@ regexp, rather than using NODE's type."
 
 (defun treesit-buffer-root-node (&optional language)
   "Return the root node of the current buffer.
-Use the first parser in `treesit-parser-list'.
 
-If optional argument LANGUAGE is non-nil, use the first parser
+Use the first parser in the parser list if LANGUAGE is omitted.
-for LANGUAGE."
+If LANGUAGE is non-nil, use the first parser for LANGUAGE in the
+parser list, or create one if none exists."
   (if-let ((parser
             (if language
                 (treesit-parser-create language)