message: MatrixGraph: add missing docs
diff: //! - [`GraphMap`](./graphmap/struct.GraphMap.html) is an adjacency list graph
  //! which is backed by a hash table and the node identifiers are the keys
  //! into the table.
  +//!
  +//! - [`MatrixGraph`](./matrix_graph/struct.MatrixGraph.html) is an adjacency matrix graph.
  +//!
  //! - [`CSR`](./csr/struct.Csr.html) is a sparse adjacency matrix graph with
  //! arbitrary associated data.
  //!
  