Improve documentation of 'window-scroll-functions'

* doc/lispref/windows.texi (Window Hooks): Clarify the values of
arguments of 'window-scroll-functions' functions.

* src/xdisp.c (syms_of_xdisp) <window-scroll-functions>: Doc fix.

doc/lispref/windows.texi

@@ -5127,7 +5127,10 @@ redisplaying a window with scrolling.  Displaying a different buffer in
 the window also runs these functions.
 
 This variable is not a normal hook, because each function is called with
-two arguments: the window, and its new display-start position.
+two arguments: the window, and its new display-start position.  At the
+time of the call, the display-start position of the window argument is
+already set to its new value, and the buffer to be displayed in the
+window is already set as the current buffer.
 
 These functions must take care when using @code{window-end}
 (@pxref{Window Start and End}); if you need an up-to-date value, you

src/xdisp.c

@@ -32569,6 +32569,9 @@ display-start position.
 These functions are called whenever the `window-start' marker is modified,
 either to point into another buffer (e.g. via `set-window-buffer') or another
 place in the same buffer.
+When each function is called, the `window-start' marker of its window
+argument has been already set to the new value, and the buffer which that
+window will display is set to be the current buffer.
 Note that the value of `window-end' is not valid when these functions are
 called.