Use treesit-extra-load-path for more values of treesit-auto-install-grammar

* lisp/treesit.el (treesit-auto-install-grammar): Use the first
writable directory from 'treesit-extra-load-path' for values
'always', 'ask', 'ask-dir' (bug#79862).

* src/treesit.c (syms_of_treesit): Extend docstring.

etc/NEWS

@@ -927,7 +927,7 @@ available when such modes are turned on.
 *** 'treesit-extra-load-path' now is a customizable user option.
 The first directory in the list is used as the default directory
 to install the language grammar when 'treesit-auto-install-grammar'
-is 'ask' or 'ask-dir'.
+is 'ask', 'ask-dir' or 'always'.
 
 *** 'treesit-language-source-alist' supports keywords.
 The language and URL are mandatory, but remaining data can use keywords:

lisp/treesit.el

@@ -5462,15 +5462,24 @@ The copied query files are queries/highlights.scm."
   "Whether to install tree-sitter language grammar libraries when needed.
 This controls whether Emacs will install missing grammar libraries
 when they are needed by some tree-sitter based mode.
+
 If `always', install the grammar library without asking.
-If `ask', ask for confirmation before installing the required grammar library.
+If `ask', ask for confirmation before installing the grammar library.
 If `ask-dir', ask for confirmation and also for a directory name where
-to install the grammar library.  The directory name is also asked when
+to install the grammar library.  The selected directory name is then
-the value is `ask' and `treesit-extra-load-path' is customized to a list
+added to the list in `treesit-extra-load-path', but not saved, so
-of directories.
+it's used only within the current session.  It's advisable to
-If nil or `never' or anything else, don't install the grammar library
+customize and save `treesit-extra-load-path' manually.
-even while visiting a file in the mode that requires such grammar; this
+
-might display a warning and/or fail to turn on the mode."
+The default directory for the above three values is the first writeable
+directory from the list in `treesit-extra-load-path'.  If it's nil, then
+the grammar is installed to the standard location, the \"tree-sitter\"
+directory under `user-emacs-directory'.
+
+If the value of this variable is nil or `never' or anything else, don't
+install the grammar library even while visiting a file in the mode that
+requires such grammar; this might display a warning and/or fail to turn
+on the mode."
   :type '(choice (const :tag "Never install grammar libraries" never)
                  (const :tag "Always automatically install grammar libraries"
                         always)
@@ -5486,23 +5495,20 @@ The option `treesit-auto-install-grammar' defines whether to install
 the grammar library if it's unavailable."
   (when (treesit-available-p)
     (or (treesit-ready-p lang t)
-        (let (out-dir)
+        (let ((out-dir (or (seq-find #'file-writable-p
+                                     treesit-extra-load-path)
+                           (locate-user-emacs-file "tree-sitter"))))
           (when (or (eq treesit-auto-install-grammar 'always)
                     (and (memq treesit-auto-install-grammar '(ask ask-dir))
                          (y-or-n-p (format "\
 Tree-sitter grammar for `%s' is missing; install it?" lang))
-                         (or (and (eq treesit-auto-install-grammar 'ask)
+                         (or (eq treesit-auto-install-grammar 'ask)
-                                  ;; Still ask dir for customized path
+                             (progn
-                                  (null treesit-extra-load-path))
-                             (let ((default-out-dir
-                                    (or (seq-find #'file-writable-p
-                                                  treesit-extra-load-path)
-                                        (locate-user-emacs-file "tree-sitter"))))
                                (setq out-dir (read-directory-name
                                               (format-prompt "\
 Install grammar for `%s' to" nil lang)
-                                              default-out-dir
+                                              out-dir
-                                              treesit-extra-load-path t))
+                                              treesit-extra-load-path))
                                (add-to-list 'treesit-extra-load-path out-dir)
                                t))))
             (treesit-install-language-grammar lang out-dir)

src/treesit.c

@@ -5275,7 +5275,8 @@ Emacs first looks in the directories mentioned in this variable,
 then in the `tree-sitter' subdirectory of `user-emacs-directory', and
 then in the system default locations for dynamic libraries, in that order.
 The first writeable directory in the list is special: it's used as the
-default directory when automatically installing the language grammar.  */);
+default directory when automatically installing the language grammar
+using `treesit-ensure-installed'.  */);
   Vtreesit_extra_load_path = Qnil;
 
   DEFVAR_LISP ("treesit-thing-settings",