cleanup: add/fix comments

thread_darwin.go

@@ -5,6 +5,9 @@ import "unsafe"
 // #include <pthread.h>
 import "C"
 
+// getThreadID returns an opaque value that is unique per OS thread.
 func getThreadID() uintptr {
+	// Darwin lacks a meaningful version of gettid() so instead we use
+	// ptread_self() as a proxy.
 	return uintptr(unsafe.Pointer(C.pthread_self()))
 }