message: DOC: Add code comments for StableGraph free lists
diff: @@ -104,6 +104,14 @@ pub struct StableGraph<N, E, Ty = Directed, Ix = DefaultIx>
  g: Graph<Option<N>, Option<E>, Ty, Ix>,
  node_count: usize,
  edge_count: usize,
  +
  + // node and edge free lists (both work the same way)
  + //
  + // free_node, if not NodeIndex::end(), points to a node index
  + // that is vacant (after a deletion). The next item in the list is kept in
  + // that Node's Node.next[0] field. For Node, it's a node index stored
  + // in an EdgeIndex location, and the _into_edge()/_into_node() methods
  + // convert.
  free_node: NodeIndex<Ix>,
  free_edge: EdgeIndex<Ix>,
  }
  