Fix documentation per bug #9949.

 src/window.c (Fwindow_body_size): Mention in the doc string that the
 return value is in frame's canonical units.
 lisp/window.el (window-body-height, window-body-width): Mention in
 the doc string that the return values are in frame's canonical
 units.
 doc/lispref/windows.texi (Window Sizes): Mention in the doc string that the
 return values of `window-body-height' and `window-body-width' are
 in frame's canonical units.

2 files changed, 18 insertions, 2 deletions

diff --git a/doc/lispref/ChangeLog b/doc/lispref/ChangeLog
index 1e86e07998c..2da562bdcfb 100644
--- a/doc/lispref/ChangeLog
+++ b/doc/lispref/ChangeLog
@@ -1,3 +1,9 @@
+2011-11-04  Eli Zaretskii  <eliz@gnu.org>
+
+        * windows.texi (Window Sizes): Mention in the doc string that the
+        return values of `window-body-height' and `window-body-width' are
+        in frame's canonical units.  (Bug#9949)
+
 2011-10-30  Martin Rudalics  <rudalics@gmx.at>
 
         * windows.texi (Windows and Frames): Remove "iso-" infix from
diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index 37679aa4a0b..bd20f5d8f44 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -563,8 +563,18 @@ window's body width.
 
 @cindex body size of a window
 @cindex window body size
-The following functions retrieve height and width of the body of a live
-window:
+@cindex canonical units of window/frame size
+The following functions retrieve height and width of the body of a
+live window.  Note that the values these functions return are measured
+in @dfn{canonical units}, i.e.@: for the default frame's face.  If the
+window shows some characters with non-default face, e.g., if the font
+of some characters is larger or smaller than the default font, the
+values returned by these functions will not match the actual number of
+lines or characters per line shown in the window.  To get the actual
+number of columns and lines, move to the last character in the line
+(e.g., with @code{end-of-visual-line}) or to the last line of the
+window (e.g., with @code{window-end}), and use @code{posn-at-point} to
+find the line or column there.
 
 @defun window-body-size &optional window horizontal
 This function returns the number of lines of @var{window}'s text area.