docs: Add comment on computation of impact parameter from the doublet

Core/include/Acts/Seeding/SeedFinder.ipp

@@ -368,8 +368,15 @@ SeedFinder<external_spacepoint_t, grid_t, platform_t>::getCompatibleDoublets(
       const float uT = xNewFrame * iDeltaR2;
       const float vT = yNewFrame * iDeltaR2;
 
-      // interactionPointCut == true we apply this cut first cuts before
-      // coordinate transformation to avoid unnecessary calculations
+      // We check the interaction point by evaluating the minimal distance
+      // between the origin and the straight line connecting the two points in
+      // the doublets. Using a geometric similarity, the Im is given by
+      // yNewFrame * rM / deltaR <= m_config.impactMax
+      // However, we make here an approximation of the impact parameter
+      // which is valid under the assumption yNewFrame / xNewFrame is small
+      // The correct computation would be:
+      // yNewFrame * yNewFrame * rM * rM <= m_config.impactMax *
+      // m_config.impactMax * deltaR2
       if (std::abs(rM * yNewFrame) <= impactMax * xNewFrame) {
         // check if duplet cotTheta is within the region of interest
         // cotTheta is defined as (deltaZ / deltaR) but instead we multiply