Add section on utility functions for paths (#409)

Give short descriptions of utility functions (as mentioned in the first paragraph) that complement the main shortest path algorithms.

Author: jd-foster

docs/src/first_steps/paths_traversal.md

@@ -33,11 +33,17 @@ The following properties always hold for shortest path algorithms implemented he
 - The distance from a vertex to itself is always `0`.
 - The distance between two vertices with no connecting edge is always `Inf` or `typemax(eltype(distmx))`.
 
-The [`dijkstra_shortest_paths`](@ref), [`desopo_pape_shortest_paths`](@ref), [`floyd_warshall_shortest_paths`](@ref), [`bellman_ford_shortest_paths`](@ref), and [`yen_k_shortest_paths`](@ref) functions return path states (subtypes of `Graphs.AbstractPathState`) that contain various information about the graph learned during traversal.
+The [`dijkstra_shortest_paths`](@ref), [`desopo_pape_shortest_paths`](@ref), [`floyd_warshall_shortest_paths`](@ref), [`bellman_ford_shortest_paths`](@ref), and [`yen_k_shortest_paths`](@ref) functions return _path states_ (subtypes of `Graphs.AbstractPathState`) that contain various information about the graph learned during traversal.
 
 The corresponding state types (with the exception of `YenState`) have the following common fields:
 
 - `state.dists` holds a vector with the distances computed, indexed by source vertex.
 - `state.parents` holds a vector of parents of each vertex on the shortest paths (the parent of a source vertex is always `0`). `YenState` substitutes `.paths` for `.parents`.
 
-In addition, with the appropriate optional arguments, [`dijkstra_shortest_paths`](@ref) will return information on all possible shortest paths available from the source.
+In addition, with the appropriate optional arguments, [`dijkstra_shortest_paths`](@ref) will return information on all possible shortest paths available from the source.
+
+## Utility functions
+
+- Once a path state is found using a shortest path algorithm, some or all of the paths can be obtained using [`enumerate_paths`](@ref).
+- A longest path within a directed acyclic graph can be found with [`dag_longest_path`](@ref).
+- In the case of a graph with some edges having negative weights, the existence of a cycle whose edges sum to a negative value can be detected with [`has_negative_edge_cycle_spfa`](@ref).