Fix intersect function for ITk in SiDetElementBoundaryLink_xk (and documenting the code)

This MR fixes a bug in the evaluation of the intersection between track parameters and detector element's bounds.

The bug fixes is at ~ line 215, where the tolerance parameter was defined with a misplaced bracket, making it being wrongly bigger by ~x100.

I took the chance to rewrite part of the code making it (I hope) more readable and clarifying the math behind the several calculations.

The fix only applies to Run-4 tracking, so no changes are foreseen for Run2-3 tests.

Anyhow, the effect on Run-4 tracking should be relatively small.