Improve docstring for `org-time-stamp-inactive'

* lisp/org.el (org-time-stamp-inactive): Improve docstring.

lisp/org.el

@@ -16629,10 +16629,18 @@ non-nil."
 
 (defun org-time-stamp-inactive (&optional arg)
   "Insert an inactive time stamp.
+
 An inactive time stamp is enclosed in square brackets instead of angle
 brackets.  It is inactive in the sense that it does not trigger agenda entries,
 does not link to the calendar and cannot be changed with the S-cursor keys.
-So these are more for recording a certain time/date."
+So these are more for recording a certain time/date.
+
+If the user specifies a time like HH:MM or if this command is called with
+at least one prefix argument, the time stamp contains the date and the time.
+Otherwise, only the date is included.
+
+When called with two universal prefix arguments, insert an active time stamp
+with the current time without prompting the user."
   (interactive "P")
   (org-time-stamp arg 'inactive))