Add documentation for horizontal scroll bars and fix some minor issues.

Add documentation for horizontal scroll bars and fix some minor issues.

* buffer.c (scroll_bar_width, scroll_bar_height): Fix
doc-strings.
* window.c (Fset_window_scroll_bars): Fix doc-string.
(Fwindow_scroll_bars): Have it return what the doc-string says.

* window.el (window-full-height-p): Make it behave correctly for
minibuffer window.
(window-current-scroll-bars): Fix code.
(fit-frame-to-buffer): Use window-scroll-bar-height instead of
window-scroll-bars.
* frame.el (frame-current-scroll-bars): Fix doc-string.
* scroll-bar.el (toggle-horizontal-scroll-bar): New command.

* frames.texi (Scroll Bars): Describe use of horizontal scroll
bars.

* display.texi (Scroll Bars): Add description of horizontal scroll
bars and associated functions.
* frames.texi (Layout Parameters): Add horizontal scroll bar
entries.  Remove paragraph on "combined fringe widths".
* windows.texi (Window Sizes): Describe affects of horizontal
scroll bars on window layout and sizes.  Fix description of
window-full-height-p.
(Resizing Windows): Mention horizontal scroll bar.