Change the with-delayed-message syntax to allow future extensibility

* doc/lispref/display.texi (Progress): Document it.
* lisp/subr.el (with-delayed-message): Change the syntax to allow
future extensibility.

* lisp/net/eww.el (eww-display-html): Use it.

doc/lispref/display.texi

@@ -561,13 +561,13 @@ You can rewrite the previous example with this macro as follows:
 @end example
 @end defmac
 
-@defmac with-delayed-message timeout message body@dots{}
+@defmac with-delayed-message (timeout message) body@dots{}
 Sometimes it's unclear whether an operation will take a long time to
 execute or not, or it can be inconvenient to implement a progress
 reporter.  This macro can be used in those situations.
 
 @lisp
-(with-delayed-message 2 (format "Gathering data for %s" entry)
+(with-delayed-message (2 (format "Gathering data for %s" entry))
   (setq data (gather-data entry)))
 @end lisp
 

lisp/net/eww.el

@@ -694,7 +694,7 @@ The renaming scheme is performed in accordance with
                 (meta . eww-tag-meta)
                 (a . eww-tag-a)))))
 	(erase-buffer)
-        (with-delayed-message 2 "Rendering HTML..."
+        (with-delayed-message (2 "Rendering HTML...")
 	  (shr-insert-document document))
 	(cond
 	 (point

lisp/subr.el

@@ -6723,12 +6723,14 @@ as the variable documentation string.
        (define-keymap--define (list ,@(nreverse opts) ,@defs))
        ,@(and doc (list doc)))))
 
-(defmacro with-delayed-message (timeout message &rest body)
+(defmacro with-delayed-message (args &rest body)
   "Like `progn', but display MESSAGE if BODY takes longer than TIMEOUT seconds.
 The MESSAGE form will be evaluated immediately, but the resulting
-string will be displayed only if BODY takes longer than TIMEOUT seconds."
+string will be displayed only if BODY takes longer than TIMEOUT seconds.
-  (declare (indent 2))
+
-  `(funcall-with-delayed-message ,timeout ,message
+\(fn (timeout message) &rest body)"
+  (declare (indent 1))
+  `(funcall-with-delayed-message ,(car args) ,(cadr args)
                                  (lambda ()
                                    ,@body)))