runtime: document sudog

Change-Id: I85c0bcf02842cc32dbc9bfdcea27efe871173574
Reviewed-on: https://go-review.googlesource.com/20774Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/runtime2.go

@@ -210,6 +210,16 @@ type gobuf struct {
 	bp   uintptr // for GOEXPERIMENT=framepointer
 }
 
+// sudog represents a g in a wait list, such as for sending/receiving
+// on a channel.
+//
+// sudog is necessary because the g ↔ synchronization object relation
+// is many-to-many. A g can be on many wait lists, so there may be
+// many sudogs for one g; and many gs may be waiting on the same
+// synchronization object, so there may be many sudogs for one object.
+//
+// sudogs are allocated from a special pool. Use acquireSudog and
+// releaseSudog to allocate and free them.
 type sudog struct {
 	// The following fields are protected by the hchan.lock of the
 	// channel this sudog is blocking on. shrinkstack depends on