Documentation updates to remove a deprecated option (place_cost_exp) and add an undocumented one (place_constraint_expand)

[vaughnbetz]

place_cost_exp has been removed, but it is still in the VPR command line options -- it should be removed.

place_constraint_expand isn't documented, but it should be (unless it's a very specific option intended only for developers, which is possible).

[soheilshahrouz]

place_constraint_expand is used to generate placement constraints based on the output of the placement stage. I feel that this option was added to generate reasonable placement constraints for testing the floorplanning feature in VPR.

place_cost_exp is removed in #2863