[qu2cu] Document what costs parameter is

Lib/fontTools/qu2cu/qu2cu.py

@@ -219,6 +219,15 @@ class Solution(NamedTuple):
 
 
 def spline_to_curves(q, costs, tolerance=0.5, all_cubic=False):
+    """
+    q: quadratic spline with alternating on-curve / off-curve points.
+
+    costs: cumulative list of encoding cost of q in terms of number of
+      points that need to be encoded.  Implied on-curve points do not
+      contribute to the cost. If all points need to be encoded, then
+      costs will be range(len(q)+1).
+    """
+
     assert len(q) >= 3, "quadratic spline requires at least 3 points"
 
     # Elevate quadratic segments to cubic