Make format-prompt interpret a nil default value as "no default"

* doc/lispref/minibuf.texi (Text from Minibuffer): Document it.

* lisp/help-fns.el (describe-function): Adjust the caller.

* lisp/minibuffer.el (format-prompt): Interpret a nil default
value as "no default".

doc/lispref/minibuf.texi

@@ -437,6 +437,9 @@ passed to @code{format} (@pxref{Formatting Strings}).
 
 @code{minibuffer-default-prompt-format} can be @samp{""}, in which
 case no default values are displayed.
+
+If @var{default} is @code{nil}, there is no default value, and
+therefore no ``default value'' string is included in the result value.
 @end defun
 
 @node Object from Minibuffer

lisp/help-fns.el

@@ -151,9 +151,7 @@ When called from lisp, FUNCTION may also be a function object."
    (let* ((fn (function-called-at-point))
           (enable-recursive-minibuffers t)
           (val (completing-read
-                (if fn
-                    (format-prompt "Describe function" fn)
-                  "Describe function: ")
+                (format-prompt "Describe function" fn)
                 #'help--symbol-completion-table
                 (lambda (f) (or (fboundp f) (get f 'function-documentation)))
                 t nil nil

lisp/minibuffer.el

@@ -3863,12 +3863,16 @@ the minibuffer was activated, and execute the forms."
 If FORMAT-ARGS is nil, PROMPT is used as a plain string.  If
 FORMAT-ARGS is non-nil, PROMPT is used as a format control
 string, and FORMAT-ARGS are the arguments to be substituted into
-it.  See `format' for details."
+it.  See `format' for details.
+
+If DEFAULT is nil, no \"default value\" string is included in the
+return value."
   (concat
    (if (null format-args)
        prompt
      (apply #'format prompt format-args))
-   (format minibuffer-default-prompt-format default)
+   (and default
+        (format minibuffer-default-prompt-format default))
    ": "))
 
 (provide 'minibuffer)