documentation/svga.txt: update outdated file

Drop CONFIG_VIDEO_400_HACK info completely.
Drop CONFIG_VIDEO_RETAIN and CONFIG_VIDEO_LOCAL completely.
Drop CONFIG_VIDEO_COMPACT and CONFIG_VIDEO_VESA info completely.
Drop CONFIG_VIDEO_SVGA info since it has been removed.
Drop chapter number & section number references since they are wrong.
Drop (bad) ftp URL for 800x600 Thinkpad XF86Config.

Rename CONFIG_VIDEO_GFX_HACK to VIDEO_GFX_HACK since it is not a
Kconfig symbol. And to match the source code.

Build options are controlled by the kernel kconfig utility.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-By: Martin Mares <mj@ucw.cz>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>