Add doc.

Former-commit-id: 9eebac2df22d9e0d1dc340e4042e5817da55c114

include/igl/copyleft/cgal/extract_feature.h

@@ -50,6 +50,24 @@ namespace igl
             const double tol,
             Eigen::PlainObjectBase<DerivedE>& feature_edges);
 
+      // Inputs:
+      //   V    #V by 3 array of vertices.
+      //   F    #F by 3 array of faces.
+      //   tol  Edges with dihedral angle larger than this are considered
+      //        as features.  Angle is measured in radian.
+      //   E    #E by 2 array of directed edges.
+      //   uE   #uE by 2 array of undirected edges.
+      //   uE2E #uE list of lists mapping undirected edges to all corresponding
+      //        directed edges.
+      //
+      // Output:
+      //   feature_edges: #E by 2 array of edges.  Each edge satisfies at
+      //      least one of the following criteria:
+      //
+      //      * Edge has dihedral angle larger than tol.
+      //      * Edge is boundary.
+      //      * Edge is non-manifold (i.e. it has more than 2 adjacent
+      //        faces).
       template <
         typename DerivedV,
         typename DerivedF,