sync: add a package doc

src/sync/doc.go

@@ -0,0 +1,4 @@
+// Package sync implements synchronization primitives similar to those provided by the standard Go implementation.
+// These are not safe to access from within interrupts, or from another thread.
+// The primitives also lack any fairness guarantees, similar to channels and the scheduler.
+package sync

src/sync/mutex.go

@@ -5,8 +5,6 @@ import (
 	_ "unsafe"
 )
 
-// These mutexes assume there is only one thread of operation and cannot be accessed safely from interrupts.
-
 type Mutex struct {
 	locked  bool
 	blocked task.Stack