Document the actual derivation of the push grid query inflation

Author: Novum

Quake/sv_phys.c

@@ -215,9 +215,11 @@ static int PushGrid_GatherCandidates (const vec3_t mins, const vec3_t maxs, edic
 	if (!push_grid_valid)
 		return -1;
 
-	// riders sit exactly on top of the pusher without overlapping it, so the
-	// query box is inflated a little (the +2 on top matters, the rest covers
-	// the epsilon expansion of absboxes in SV_LinkEdict)
+	// strictly overlapping absboxes always share a cell, and riders touching the
+	// pusher overlap it through the +-1 absbox expansion in SV_LinkEdict, so no
+	// inflation is needed in theory. The +2 is a safety margin for riders whose
+	// ONGROUND/groundentity state outlives actual contact by a small gap (the
+	// elevator DIST_EPSILON nudge, float drift); it almost never adds a cell.
 	int lo[3], hi[3];
 	PushGrid_CellRange (mins, maxs, 2.0f, lo, hi);