message: Added comment to clarify semantics of opt_level.
diff: @@ -102,6 +102,8 @@ message OptimizerOptions {
  L0 = -1;
  }
  + // Overall optimization level. The actual optimizations applied will be the
  + // logical OR of the flags that this level implies and any flags already set.
  Level opt_level = 3;
  // Control the use of the compiler/jit. Experimental.
  