[BUG] Rename `hess` field into `hess_inv` for result of BFGS minimization

Also document the fact that the Hessian inverse returned by BFGS is
approximated.

Closes #2647.
DOC: optimize: warn about approximate hessians
Merge pull request #2659 from dlax/issue-2647

BUG: optimize: Rename `hess` field into `hess_inv` for result of BFGS minimization

Also warn in the documentation that these may be just approximations.