Add docs for generators and generators::petersen

src/generators.rs

@@ -1,5 +1,13 @@
+//! Generators for common graphs.
+
 use crate::traits::{GraphTopology, GraphTopologyAddition};
 
+/// Generates a Petersen graph.
+///
+/// Generates a [Petersen graph] in `graph` as a new connected component and returns the new
+/// vertices and edges. This graph is also the Kneser graph *KG(5, 2)*.
+///
+/// [Petersen graph]: https://en.wikipedia.org/wiki/Petersen_graph
 pub fn petersen<G>(
     graph: &mut G,
 ) -> (