Fix minor issues with removing left or right fringes

Fix minor issues with removing left or right fringes

* lisp/window.el (window-max-chars-per-line): Account for
'left-fringe-width' and 'right-fringe-width' variables.

* doc/lispref/windows.texi (Window Sizes): Document the effect on
window text width when setting the width of one or both fringes to
zero.
* doc/emacs/display.texi (Fringes): Document the effect on window
text width when setting the width of one or both fringes to zero.
(Bug#22891)