Edit docs for rustc_data_structures::graph::scc

- Add newline to provide concise module summary
- Add wikipedia link
- Italicize O notation

src/librustc_data_structures/graph/scc/mod.rs

@@ -1,7 +1,9 @@
-//! Routine to compute the strongly connected components (SCCs) of a
-//! graph, as well as the resulting DAG if each SCC is replaced with a
-//! node in the graph. This uses Tarjan's algorithm that completes in
-//! O(n) time.
+//! Routine to compute the strongly connected components (SCCs) of a graph.
+//!
+//! Also computes as the resulting DAG if each SCC is replaced with a
+//! node in the graph. This uses [Tarjan's algorithm](
+//! https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm)
+//! that completes in *O(n)* time.
 
 use crate::fx::FxHashSet;
 use crate::graph::vec_graph::VecGraph;