'C-u C-x .' clears the fill prefix

* lisp/textmodes/fill.el (set-fill-prefix): When called
interactively with a prefix argument, clear the fill prefix,
just like how 'C-u C-x C-n' clears the goal column.
* doc/emacs/text.texi (Fill Prefix):
* etc/NEWS: Document the change.

doc/emacs/text.texi

@@ -699,8 +699,9 @@ a new paragraph.
   To specify a fill prefix for the current buffer, move to a line that
 starts with the desired prefix, put point at the end of the prefix,
 and type @w{@kbd{C-x .}}@: (@code{set-fill-prefix}).  (That's a period
-after the @kbd{C-x}.)  To turn off the fill prefix, specify an empty
-prefix: type @w{@kbd{C-x .}}@: with point at the beginning of a line.
+after the @kbd{C-x}.)  To turn off the fill prefix, either type
+@w{@kbd{C-u C-x .}}@: or specify an empty prefix: type @w{@kbd{C-x .}}@:
+with point at the beginning of a line.
 
   When a fill prefix is in effect, the fill commands remove the fill
 prefix from each line of the paragraph before filling, and insert it

etc/NEWS

@@ -619,6 +619,11 @@ sentence.  The new command 'fill-region-as-paragraph-semlf' fills a
 region of text using semantic linefeeds as if the region were a single
 paragraph.
 
++++
+** 'C-u C-x .' clears the fill prefix.
+You can now use 'C-u C-x .' to clear the fill prefix, similarly to how
+you could already use 'C-u C-x C-n' to clear the goal column.
+
 
 * Changes in Specialized Modes and Packages in Emacs 31.1
 

lisp/textmodes/fill.el

@@ -78,21 +78,24 @@ placed at the beginning or end of a line by filling.
 See the documentation of `kinsoku' for more information."
   :type 'boolean)
 
-(defun set-fill-prefix ()
+(defun set-fill-prefix (&optional arg)
   "Set the fill prefix to the current line up to point.
 Filling expects lines to start with the fill prefix and
-reinserts the fill prefix in each resulting line."
-  (interactive)
-  (let ((left-margin-pos (save-excursion (move-to-left-margin) (point))))
-    (if (> (point) left-margin-pos)
-	(progn
-	  (setq fill-prefix (buffer-substring left-margin-pos (point)))
-	  (if (equal fill-prefix "")
+reinserts the fill prefix in each resulting line.
+With a prefix argument, cancel the fill prefix."
+  (interactive "P")
+  (if arg
+      (setq fill-prefix nil)
+    (let ((left-margin-pos (save-excursion (move-to-left-margin) (point))))
+      (if (> (point) left-margin-pos)
+	  (progn
+	    (setq fill-prefix (buffer-substring left-margin-pos (point)))
+	    (when (equal fill-prefix "")
 	      (setq fill-prefix nil)))
-      (setq fill-prefix nil)))
+        (setq fill-prefix nil))))
   (if fill-prefix
       (message "fill-prefix: \"%s\"" fill-prefix)
-    (message "fill-prefix canceled")))
+    (message "fill-prefix cancelled")))
 
 (defcustom adaptive-fill-mode t
   "Non-nil means determine a paragraph's fill prefix from its text."