[PATCH] doc: more serial-console info

Add info on flow control for serial consoles.  Refer to netconsole option
also.

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>