runtime: fix another out of date comment in GC

runtime: fix another out of date comment in GC

gcDrain used to be passed a *workbuf to start draining from, but now
it takes a gcWork, which hides whether or not there's an initial
workbuf. Update the comment to match this.

Change-Id: I976b58e5bfebc451cfd4fa75e770113067b5cc07
Reviewed-on: https://go-review.googlesource.com/8246
Reviewed-by: Rick Hudson <rlh@golang.org>