doc/syslinux.txt: grammar

Reported-by: Ady <ady-sf@hotmail.com>
Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
txt/syslinux.cfg.txt: Add SYSAPPEND

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
txt/syslinux.cfg.txt: Add SENDCOOKIES, example config

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
txt/syslinux-cli.txt: Path rules

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
txt/syslinux.txt: synopsis, extlinux.sys, wrap long command

Add -v/--version to synopsis; add note on extlinux.sys from old
versions; Wrap altmbr command for cleaner manpage

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
txt/syslinux.cfg.txt: Updates

DEFAULT often references a LABEL; TIMEOUT action affected by UI.
IPAPPEND/SYSAPPEND: wrap SYSAPPEND; notes on mask values 2,1.
TIMEOUT-related clarifications
SERIAL is sticky
PATH updated for 5.10-next with notes

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
txt/: Add common file for derivatives

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