Fix "C-u M-!" when 'shell-command-dont-erase-buffer' is non-nil

* lisp/simple.el (shell-command-dont-erase-buffer): Clarify the
effect of the various values in the doc string.
(shell-command-save-pos-or-erase, shell-command): Don't move or
push point if the output will go to the current buffer.
(Bug#40690)
(shell-command): Mention 'shell-command-dont-erase-buffer' in the
doc string.

* test/lisp/simple-tests.el
(with-shell-command-dont-erase-buffer): Don't is shell quoting
'like this', as it doesn't work on MS-Windows; quote "like this"
instead.
(simple-tests-shell-command-dont-erase-buffer): Adapt the test to
the new modus operandi.

* doc/emacs/misc.texi (Single Shell): Document the effect of the
various values of 'shell-command-dont-erase-buffer'.

* etc/NEWS: Expand and reword the entry regarding changes in
'shell-command-dont-erase-buffer'.

lisp/simple.el

@@ -3437,20 +3437,22 @@ This affects `shell-command' and `async-shell-command'."
   :version "27.1")
 
 (defcustom shell-command-dont-erase-buffer nil
-  "Control if the output buffer is erased before the command.
+  "Whether to erase the output buffer before executing shell command.
 
 A nil value erases the output buffer before execution of the
 shell command, except when the output buffer is the current one.
 
 The value `erase' ensures the output buffer is erased before
-execution of the shell command.
+execution of the shell command even if it is the current buffer.
 
-Other non-nil values prevent the output buffer from being erased and
-set the point after execution of the shell command.
+Other non-nil values prevent the output buffer from being erased; they
+also reposition point in the shell output buffer after execution of the
+shell command, except when the output buffer is the current buffer.
 
-The value `beg-last-out' sets point at the beginning of the output,
-`end-last-out' sets point at the end of the buffer, `save-point'
-restores the buffer position before the command."
+The value `beg-last-out' sets point at the beginning of the last
+output, `end-last-out' sets point at the end of the last output,
+and `save-point' restores the buffer position as it was before the
+shell command."
   :type '(choice
           (const :tag "Erase output buffer if not the current one" nil)
           (const :tag "Always erase output buffer" erase)
@@ -3480,9 +3482,12 @@ See `shell-command-dont-erase-buffer'."
     ;; if some text has a non-nil read-only property,
     ;; which comint sometimes adds for prompts.
     (setq pos
-          (cond ((eq sym 'save-point) (point))
-                ((eq sym 'beg-last-out) (point-max))
-                ;;((not sym)
+          (cond ((eq sym 'save-point)
+                 (if (not output-to-current-buffer)
+                     (point)))
+                ((eq sym 'beg-last-out)
+                 (if (not output-to-current-buffer)
+                     (point-max)))
                 ((or (eq sym 'erase)
                      (and (null sym) (not output-to-current-buffer)))
                  (let ((inhibit-read-only t))
@@ -3597,10 +3602,15 @@ says to put the output in some other buffer.
 If OUTPUT-BUFFER is a buffer or buffer name, erase that buffer
 and insert the output there; a non-nil value of
 `shell-command-dont-erase-buffer' prevents the buffer from being
-erased.  If OUTPUT-BUFFER is not a buffer and not nil, insert the
+erased.  If OUTPUT-BUFFER is not a buffer and not nil (which happens
+interactively when the prefix argument is given), insert the
 output in current buffer after point leaving mark after it.  This
 cannot be done asynchronously.
 
+The user option `shell-command-dont-erase-buffer', which see, controls
+whether the output buffer is erased and where to put point after
+the shell command.
+
 If the command terminates without error, but generates output,
 and you did not specify \"insert it in the current buffer\",
 the output can be displayed in the echo area or in its buffer.
@@ -3688,8 +3698,7 @@ impose the use of a shell (with its need to quote arguments)."
 	    ;; because we inserted text.
 	    (goto-char (prog1 (mark t)
 			 (set-marker (mark-marker) (point)
-				     (current-buffer))))
-            (shell-command-set-point-after-cmd))
+				     (current-buffer)))))
 	;; Output goes in a separate buffer.
 	;; Preserve the match data in case called from a program.
         ;; FIXME: It'd be ridiculous for an Elisp function to call