Cleanup/docs: Add comment to spline lookup factor method

source/blender/blenkernel/BKE_spline.hh

@@ -196,9 +196,16 @@ class Spline {
      */
     float factor;
   };
+  /**
+   * Find the position on the evaluated spline at the given portion of the total length.
+   * The return value is the indices of the two neighboring points at that location and the
+   * factor between them, which can be used to look up any attribute on the evaluated points.
+   * \note This does not support extrapolation.
+   */
   LookupResult lookup_evaluated_factor(const float factor) const;
   /**
-   * \note This does not support extrapolation currently.
+   * The same as #lookup_evaluated_factor, but looks up a length directly instead of
+   * a portion of the total.
    */
   LookupResult lookup_evaluated_length(const float length) const;