runtime: make the GC message less committal.

We shouldn't guarantee this behavior, but suggest it's possible.

Change-Id: I4c2afb48b99be4d91537306d3337171a13c9990a
Reviewed-on: https://go-review.googlesource.com/12346
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/mgc.go

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