message: Trivial: update clang thread-safety docs url
diff: // TL;DR Add GUARDED_BY(mutex) to member variables. The others are
  // rarely necessary. Ex: int nFoo GUARDED_BY(cs_foo);
  //
  -// See http://clang.llvm.org/docs/LanguageExtensions.html#threadsafety
  +// See https://clang.llvm.org/docs/ThreadSafetyAnalysis.html
  // for documentation. The clang compiler can do advanced static analysis
  // of locking when given the -Wthread-safety option.
  #define LOCKABLE __attribute__((lockable))
  