runtime: clarify runtime.GC blocking behavior

The runtime.GC documentation was rewritten in df2809f0 to make it clear
that it blocks until GC is complete, but the re-rewrite in ed9a4c91 and
e28a6792 lost this property when clarifying that it may also block the
entire program and not just the caller.

Try to arrive at wording that conveys both of these properties.

Change-Id: I1e255322aa28a21a548556ecf2a44d8d8ac524ef
Reviewed-on: https://go-review.googlesource.com/12392Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/runtime/mgc.go

@@ -770,8 +770,9 @@ var work struct {
 	initialHeapLive uint64
 }
 
-// GC runs a garbage collection. It might block the entire program until the
-// collection is complete.
+// GC runs a garbage collection and blocks the caller until the
+// garbage collection is complete. It may also block the entire
+// program.
 func GC() {
 	startGC(gcForceBlockMode)
 }