Clarify a few comments in TR

These are for letrec typechecking and Tarjan's algorithm

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/typecheck/tc-let-unit.rkt

@@ -189,8 +189,9 @@
 (struct lr-clause (names expr clause) #:transparent)
 
 ;; get-non-recursive-clauses : (Listof lr-clause) (Listof Identifier) ->
-;;                             (Listof lr-clause) (Listof )
-;; Find letrec-values clauses that do not create variable cycles
+;;                             (Listof lr-clause) (Listof lr-clause)
+;; Find letrec-values clauses that do not create variable cycles. Return
+;; both the non-recursive clauses and the remaining recursive ones.
 (define (get-non-recursive-clauses clauses flat-names)
 
   ;; Set up vertices for Tarjan's algorithm, where each letrec-values
@@ -215,7 +216,8 @@
       (andmap (λ (id2) (not (bound-identifier=? id id2)))
               (vertex-adjacent vertex))))
 
-  ;; The components with only one entry are non-recursive
+  ;; The components with only one entry are non-recursive if they also
+  ;; contain no self-cycles.
   (for/fold ([non-recursive '()]
              [remaining '()])
             ([component components])

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/utils/tarjan.rkt

@@ -35,6 +35,10 @@
 ;; tarjan : (Dict K (Vertex K V)) -> (Listof (Listof (Vertex K V)))
 ;; Implements Tarjan's algorithm. See Wikipedia
 ;; http://en.wikipedia.org/wiki/Tarjan's_strongly_connected_components_algorithm
+;;
+;; Note that because this accumulates the result with `cons`, the components
+;; are actually output in topological order rather than *reverse* topological order.
+;; If you want reverse topological order, reverse the result.
 (define (tarjan vertices)
   (define (strongly-connected vtx)
     (set-vertex-index! vtx index)