Document order of multiple solutions

[odow]

Closes #1372

[blegat]

If multiple results are present, the first result (i.e., result_index=1) must be an optimal solution.

This disallows having multiple results in case the termination status is not OPTIMAL.
I would rather say something like

If the termination status is OPTIMAL, and multiple results are present, the first result (i.e., result_index=1) must be an optimal solution.
If the termination status is INFEASIBLE and one of the results is an infeasibility certificate, it should be first result.
If the termination status is DUAL_INFEASIBLE and one of the results is a dual infeasibility certificate, it should be the first result.
If the termination status is LOCALLY_SOLVED and one of the results is a stationary point or local optimal solution, it should be the first result.

At least that's what we assume in the tests so it makes sense to make it a requirements.

[odow]

How's this now.