message: [XLA] Clarify that options passed to LocalService override legacy_flags values.
diff: @@ -58,12 +58,18 @@ class LocalExecutable {
  // Validates that the given arguments and options satisfy various constraints
  // of the computation.
  + //
  + // The given ExecutableRunOptions override any values from legacy_flags
  + // (TF_XLA_FLAGS environment variable).
  Status ValidateExecutionOptions(
  const tensorflow::gtl::ArraySlice<const ShapedBuffer*> arguments,
  const ExecutableRunOptions& run_options, const Backend& backend);
  // Records the computation in a SessionModule proto with the arguments used to
  // invoke it, and the result. Enabled by flag: --tla_dump_executions_to.
  + //
  + // The given ServiceExecutableRunOptions override any values from legacy_flags
  + // (TF_XLA_FLAGS environment variable).
  StatusOr<ScopedShapedBuffer> ExecuteAndDump(
  const ServiceExecutableRunOptions* run_options,
  const tensorflow::gtl::ArraySlice<const ShapedBuffer*> arguments);
  @@ -109,6 +115,9 @@ class LocalClient : public Client {
  // Build and return a LocalExecutable object. The executable is compiled using
  // the given XlaComputation, argument layouts and options.
  + //
  + // The given ExecutableBuildOptions override any values from legacy_flags
  + // (TF_XLA_FLAGS environment variable).
  StatusOr<std::unique_ptr<LocalExecutable>> Compile(
  const XlaComputation& computation,
  const tensorflow::gtl::ArraySlice<const Shape*> argument_layouts,
  