Improve docs of 'posn-actual-col-row' (Bug#20169)

 doc/lispref/commands.texi (Accessing Mouse): Expand documentation of
 'posn-actual-col-row'.

doc/lispref/ChangeLog

@@ -2,6 +2,8 @@
 
 	* commands.texi (Event Input Misc): Fix incorrect usage of @code.
 	(Bug#20174)
+	(Accessing Mouse): Expand documentation of 'posn-actual-col-row'.
+	(Bug#20169)
 
 2015-03-14  Eli Zaretskii  <eliz@gnu.org>
 

doc/lispref/commands.texi

@@ -2081,7 +2081,13 @@ Return the actual row and column in @var{position}, as a cons cell
 @code{(@var{col} . @var{row})}.  The values are the actual row and
 column numbers in the window given by @var{position}.  @xref{Click
 Events}, for details.  The function returns @code{nil} if
-@var{position} does not include actual position values.
+@var{position} does not include actual position values; in that case
+@code{posn-col-row} can be used to get approximate values.
+
+Note that this function doesn't account for the visual width of
+characters on display, like the number of visual columns taken by a
+tab character or an image.  If you need the coordinates in canonical
+character units, use @code{posn-col-row} instead.
 @end defun
 
 @defun posn-string position