Fix bug #15375 with inaccurate docs of display margin width values.

 doc/lispref/display.texi (Display Margins): State the units of measuring
 margin width.

 src/buffer.c (syms_of_buffer) <left-margin-width, right-margin-width>:
 Doc fix.

doc/lispref/ChangeLog

+2013-09-14  Eli Zaretskii  <eliz@gnu.org>
+
+	* display.texi (Display Margins): State the units of measuring
+	margin width.  (Bug#15375)
+
 2013-09-13  Eli Zaretskii  <eliz@gnu.org>
 
 	* text.texi (Not Intervals): Minor wording fix.

doc/lispref/display.texi

@@ -4189,13 +4189,15 @@ them a nonzero width.  The usual way to do that is to set these
 variables:
 
 @defvar left-margin-width
-This variable specifies the width of the left margin.
-It is buffer-local in all buffers.
+This variable specifies the width of the left margin, in character
+cell units.  It is buffer-local in all buffers.  A value of @code{nil}
+means no left marginal area.
 @end defvar
 
 @defvar right-margin-width
-This variable specifies the width of the right margin.
-It is buffer-local in all buffers.
+This variable specifies the width of the right margin, in character
+cell units.  It is buffer-local in all buffers.  A value of @code{nil}
+means no right marginal area.
 @end defvar
 
   Setting these variables does not immediately affect the window.  These
@@ -4206,15 +4208,19 @@ Thus, you can make changes take effect by calling
   You can also set the margin widths immediately.
 
 @defun set-window-margins window left &optional right
-This function specifies the margin widths for window @var{window}.
-The argument @var{left} controls the left margin and
-@var{right} controls the right margin (default @code{0}).
+This function specifies the margin widths for window @var{window}, in
+character cell (a.k.a.@: ``column''), units.  The argument @var{left}
+controls the left margin and @var{right} controls the right margin
+(default @code{0}).
 @end defun
 
 @defun window-margins &optional window
-This function returns the left and right margins of @var{window}
-as a cons cell of the form @code{(@var{left} . @var{right})}.
-If @var{window} is @code{nil}, the selected window is used.
+This function returns the width of the left and right margins of
+@var{window} as a cons cell of the form @w{@code{(@var{left}
+. @var{right})}}.  If one of the two marginal areas does not exist,
+its width is returned as @code{nil}; if none of the two margins exist,
+the function returns @code{nil}.  If @var{window} is @code{nil}, the
+selected window is used.
 @end defun
 
 @node Images

src/ChangeLog

+2013-09-14  Eli Zaretskii  <eliz@gnu.org>
+
+	* buffer.c (syms_of_buffer) <left-margin-width,	right-margin-width>:
+	Doc fix.  (Bug#15375)
+
 2013-09-13  Dmitry Antipov  <dmantipov@yandex.ru>
 
 	Unify Fx_focus_frame between all ports.

src/buffer.c

@@ -5900,7 +5900,7 @@ See also the functions `display-table-slot' and `set-display-table-slot'.  */);
 
   DEFVAR_PER_BUFFER ("left-margin-width", &BVAR (current_buffer, left_margin_cols),
 		     Qintegerp,
-		     doc: /* Width of left marginal area for display of a buffer.
+		     doc: /* Width in columns of left marginal area for display of a buffer.
 A value of nil means no marginal area.
 
 Setting this variable does not take effect until a new buffer is displayed
@@ -5908,7 +5908,7 @@ in a window.  To make the change take effect, call `set-window-buffer'.  */);
 
   DEFVAR_PER_BUFFER ("right-margin-width", &BVAR (current_buffer, right_margin_cols),
 		     Qintegerp,
-		     doc: /* Width of right marginal area for display of a buffer.
+		     doc: /* Width in columns of right marginal area for display of a buffer.
 A value of nil means no marginal area.
 
 Setting this variable does not take effect until a new buffer is displayed