Document behaviour of QP solver if an indefinite Hessian is passed

[jajhall]

Further to rust-or/highs#44, example

min x^2 + 4xy + y^2 s.t. x + y = 1 (variables free)

is non-convex.

[jajhall]

@filikat The assertion in HiPO is exposed in test2 of the unit test issue-3045.

[jajhall]

Closed by #3056