message: proper reference
diff: //! This implementation follows the excellent MATLAB implementation of Dianne P. O'Leary at
  //! http://www.cs.umd.edu/users/oleary/software/
  //!
  +//! ## Reference
  +//!
  +//! Jorge J. More and David J. Thuente. "Line search algorithms with guaranteed sufficient
  +//! decrease." ACM Trans. Math. Softw. 20, 3 (September 1994), 286-307.
  +//! DOI: https://doi.org/10.1145/192115.192132
  +//!
  // //!
  // //! # Example
  // //!
  