The big documentation comment at the top of malloc.go has gotten
woefully out of date. Update it.

Change-Id: Ibdb1bdcfdd707a6dc9db79d0633a36a28882301b
Reviewed-on: https://go-review.googlesource.com/29731Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Rick Hudson <rlh@golang.org>