Decreasing update threshold for BFGS as per L-BFGS.

- Improves performance of BFGS on NIST, as per L-BFGS.
- Adding explanation of origin and purpose of Secant condition
  tolerance check for Hessian update in (L)BFGS.

Change-Id: If57b9957d31d8629c772c19a069e1e56e727b350