[dev.garbage] runtime: document sysAlloc

In particular, it always returns an aligned pointer.

Change-Id: I763789a539a4bfd8b0efb36a39a80be1a479d3e2
Reviewed-on: https://go-review.googlesource.com/22558


Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>