docs: ramdisk/initrd/initramfs corrections

initrd/initramfs/ramdisk docs:
- fix typos/spellos/grammar
- clarify RAM disk config location
- correct cpio option

Acked-by: Bryan O'Sullivan <bos@serpentine.com>
Acked-by: Rob Landley <rob@landley.net>
Cc: Werner Almesberger <werner@almesberger.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>