Document restrictions when setting window margins, fringes or scroll bars

* src/window.c (Fset_window_margins, Fset_window_fringes)
(Fset_window_scroll_bars): In doc-strings tell that a window
must be large enough to accommodate fringes, sroll bars and
margins of the desired size.
* doc/lispref/display.texi (Fringe Size/Pos, Scroll Bars)
(Display Margins): Tell that windows must be large enough to
accommodate fringes, sroll bars and margins of the desired
size.

doc/lispref/display.texi

@@ -3916,6 +3916,9 @@ fringe, and likewise @var{right} for the right fringe.  A value of
 @var{outside-margins} is non-@code{nil}, that specifies that fringes
 should appear outside of the display margins.
 
+If @var{window} is not large enough to accommodate fringes of the
+desired width, this leaves the fringes of @var{window} unchanged.
+
 The values specified here may be later overridden by invoking
 @code{set-window-buffer} (@pxref{Buffers and Windows}) on @var{window}
 with its @var{keep-margins} argument @code{nil} or omitted.
@@ -4337,6 +4340,9 @@ vertical scroll bar.
 The possible values are @code{bottom}, @code{t}, which means to use the
 frame's default, and @code{nil} for no horizontal scroll bar.
 
+If @var{window} is not large enough to accommodate a scroll bar of the
+desired dimension, this leaves the corresponding scroll bar unchanged.
+
 The values specified here may be later overridden by invoking
 @code{set-window-buffer} (@pxref{Buffers and Windows}) on @var{window}
 with its @var{keep-margins} argument @code{nil} or omitted.
@@ -4926,6 +4932,9 @@ This function specifies the margin widths for window @var{window}, in
 character cell units.  The argument @var{left} controls the left
 margin, and @var{right} controls the right margin (default @code{0}).
 
+If @var{window} is not large enough to accommodate margins of the
+desired width, this leaves the margins of @var{window} unchanged.
+
 The values specified here may be later overridden by invoking
 @code{set-window-buffer} (@pxref{Buffers and Windows}) on @var{window}
 with its @var{keep-margins} argument @code{nil} or omitted.

src/window.c

@@ -7076,7 +7076,9 @@ reserve for the left marginal area.  Optional third arg RIGHT-WIDTH
 does the same for the right marginal area.  A nil width parameter
 means no margin.
 
-Return t if any margin was actually changed and nil otherwise.  */)
+Leave margins unchanged if WINDOW is not large enough to accommodate
+margins of the desired width.  Return t if any margin was actually
+changed and nil otherwise.  */)
   (Lisp_Object window, Lisp_Object left_width, Lisp_Object right_width)
 {
   struct window *w = set_window_margins (decode_live_window (window),
@@ -7158,11 +7160,14 @@ the left fringe.  Optional third arg RIGHT-WIDTH specifies the right
 fringe width.  If a fringe width arg is nil, that means to use the
 frame's default fringe width.  Default fringe widths can be set with
 the command `set-fringe-style'.
+
 If optional fourth arg OUTSIDE-MARGINS is non-nil, draw the fringes
 outside of the display margins.  By default, fringes are drawn between
 display marginal areas and the text area.
 
-Return t if any fringe was actually changed and nil otherwise.  */)
+Leave fringes unchanged if WINDOW is not large enough to accommodate
+fringes of the desired width.  Return t if any fringe was actually
+changed and nil otherwise.  */)
   (Lisp_Object window, Lisp_Object left_width,
    Lisp_Object right_width, Lisp_Object outside_margins)
 {
@@ -7287,6 +7292,8 @@ horizontal scroll bar: bottom, nil, or t where nil means to not display
 a horizontal scroll bar on WINDOW and t means to use WINDOW frame's
 horizontal scroll bar type.
 
+If WINDOW is not large enough to accommodate a scroll bar of the
+desired dimension, leave the corresponding scroll bar unchanged.
 Return t if scroll bars were actually changed and nil otherwise.  */)
   (Lisp_Object window, Lisp_Object width, Lisp_Object vertical_type,
    Lisp_Object height, Lisp_Object horizontal_type)