txt/syslinux-cli.txt: Version on Ctrl-N

Only on some versions.

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
txt/: Add isolinux.txt, pxelinux.txt

Reformatted from previous in doc/.  Reflow to try to be more
manpage-like and put more commonly used information on the top.

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
txt/Makefile: add isolinux.txt, pxelinux.txt

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>