Document restrictions when setting window margins, fringes or scroll bars

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.