Minor updates to specialization subchapter

Author: mark-i-m

src/trait-specialization.md

@@ -1,9 +1,12 @@
 # Specialization
 
+**TODO**: where does Chalk fit in? Should we mention/discuss it here?
+
 Defined in the `specialize` module.
 
 The basic strategy is to build up a *specialization graph* during
-coherence checking. Insertion into the graph locates the right place
+coherence checking (recall that coherence checking looks for overlapping
+impls). Insertion into the graph locates the right place
 to put an impl in the specialization hierarchy; if there is no right
 place (due to partial overlap but no containment), you get an overlap
 error. Specialization is consulted when selecting an impl (of course),