Update doc for helm-M-x

helm-command.el

@@ -51,9 +51,13 @@ Show all candidates on startup when 0 (default)."
 
 (defcustom helm-M-x-allow-prefix-argument t
   "Allow specifying prefix argument before `helm-M-x' when non--nil.
-Note if you set it to non--nil and specify a prefix argument
-before `helm-M-x', then you will NOT be able to cancel it though
-you can change it to another vaule."
+
+If this variable is non--nil and you specify prefix argument before `helm-M-x',
+the first C-u after `helm-M-x' will be used to clear the initial prefix
+argument, otherwise C-u works as usual.
+
+If this variable is nil, inhibits setting prefix argument before `helm-M-x'
+by signaling an error."
   :group 'helm-command
   :type 'boolean)
 

helm-help.el

@@ -1009,11 +1009,12 @@ This feature is only available with emacs-25.
 
 *** Prefix Args
 
-All the prefix args passed BEFORE running `helm-M-x' are ignored,
-you should get an error message if you do so.
-When you want to pass prefix args, pass them AFTER starting `helm-M-x',
+When you want pass prefix args, you should pass prefix args AFTER starting `helm-M-x',
 you will see a prefix arg counter appearing in mode-line notifying you
-the number of prefix args entered.")
+the number of prefix args entered.
+
+If you pass prefix args before running `helm-M-x', it will be displayed in prompt,
+then the first C-u after `helm-M-x' will be used to clear that prefix args.")
 
 ;;; helm-imenu
 ;;