Improve documentation of set-window-start

* doc/lispref/windows.texi (Window Start and End):
* src/window.c (Fset_window_start): Document that reliable
setting of a window start position requires to adjust point to
be visible.  (Bug#34038)

doc/lispref/windows.texi

@@ -4625,13 +4625,14 @@ This function sets the display-start position of @var{window} to
 @var{position} in @var{window}'s buffer.  It returns @var{position}.
 
 The display routines insist that the position of point be visible when a
-buffer is displayed.  Normally, they change the display-start position
-(that is, scroll the window) whenever necessary to make point visible.
-However, if you specify the start position with this function using
-@code{nil} for @var{noforce}, it means you want display to start at
-@var{position} even if that would put the location of point off the
-screen.  If this does place point off screen, the display routines move
-point to the left margin on the middle line in the window.
+buffer is displayed.  Normally, they select the display-start position
+according to their internal logic (and scroll the window if necessary)
+to make point visible.  However, if you specify the start position
+with this function using @code{nil} for @var{noforce}, it means you
+want display to start at @var{position} even if that would put the
+location of point off the screen.  If this does place point off
+screen, the display routines attempt to move point to the left margin
+on the middle line in the window.
 
 For example, if point @w{is 1} and you set the start of the window
 @w{to 37}, the start of the next line, point will be above the top
@@ -4678,6 +4679,13 @@ it is still 1 when redisplay occurs.  Here is an example:
 @end group
 @end example
 
+If the attempt to make point visible (i.e., in a fully-visible screen
+line) fails, the display routines will disregard the requested
+window-start position and compute a new one anyway.  Thus, for
+reliable results Lisp programs that call this function should always
+move point to be inside the window whose display starts at
+@var{position}.
+
 If @var{noforce} is non-@code{nil}, and @var{position} would place point
 off screen at the next redisplay, then redisplay computes a new window-start
 position that works well with point, and thus @var{position} is not used.

src/window.c

@@ -1704,7 +1704,12 @@ DEFUN ("set-window-start", Fset_window_start, Sset_window_start, 2, 3, 0,
        doc: /* Make display in WINDOW start at position POS in WINDOW's buffer.
 WINDOW must be a live window and defaults to the selected one.  Return
 POS.  Optional third arg NOFORCE non-nil inhibits next redisplay from
-overriding motion of point in order to display at this exact start.  */)
+overriding motion of point in order to display at this exact start.
+
+For reliable setting of WINDOW start position, make sure point is
+at a position that will be visible when that start is in effect,
+otherwise there's a chance POS will be disregarded, e.g., if point
+winds up in a partially-visible line.  */)
   (Lisp_Object window, Lisp_Object pos, Lisp_Object noforce)
 {
   register struct window *w = decode_live_window (window);