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.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/window.c b/src/window.c
index 42a2ca68f14..0f35066c7d5 100644
--- a/src/window.c
+++ b/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]),