runtime: document that GC blocks the whole program

No code changes. Just make it clear that runtime.GC is not concurrent.

Change-Id: I00a99ebd26402817c665c9a128978cef19f037be
Reviewed-on: https://go-review.googlesource.com/12345Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/mgc.go

@@ -770,7 +770,7 @@ var work struct {
 	initialHeapLive uint64
 }
 
-// GC runs a garbage collection and blocks until the garbage
+// GC runs a garbage collection and blocks the entire program until the
 // collection is complete.
 func GC() {
 	startGC(gcForceBlockMode)