documentation: Documentation/initrd.txt

Final clearification of the pivot_root mechanism, which brings this
document really up-to-date.

Signed-off-by: Domenico Andreoli <cavok@dandreoli.com>
Cc: "Randy.Dunlap" <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>