Clarify documentation of pos-visible-in-window-p

* src/window.c (Fpos_visible_in_window_p): Clarify the meaning of
t for POS.  See
http://lists.gnu.org/archive/html/emacs-devel/2015-09/msg01040.html
for the original report.

* doc/lispref/windows.texi (Window Start and End): Clarify the
meaning of t for the POSITION argument of pos-visible-in-window-p.

doc/lispref/windows.texi

@@ -3179,7 +3179,7 @@ Locations that are partially obscured are not considered visible unless
 @var{partially} is non-@code{nil}.  The argument @var{position} defaults
 to the current position of point in @var{window}; @var{window}, to the
 selected window.  If @var{position} is @code{t}, that means to check the
-last visible position in @var{window}.
+first visible position of the last visible screen line in @var{window}.
 
 This function considers only vertical scrolling.  If @var{position} is
 out of view only because @var{window} has been scrolled horizontally,

src/window.c

@@ -1686,8 +1686,9 @@ Return nil if that position is scrolled vertically out of view.  If a
 character is only partially visible, nil is returned, unless the
 optional argument PARTIALLY is non-nil.  If POS is only out of view
 because of horizontal scrolling, return non-nil.  If POS is t, it
-specifies the position of the last visible glyph in WINDOW.  POS
-defaults to point in WINDOW; WINDOW defaults to the selected window.
+specifies the position of the first glyph displayed on the last visible
+screen line in WINDOW.  POS defaults to point in WINDOW; WINDOW defaults
+to the selected window.
 
 If POS is visible, return t if PARTIALLY is nil; if PARTIALLY is non-nil,
 the return value is a list of 2 or 6 elements (X Y [RTOP RBOT ROWH VPOS]),