Add guide documentation on printing exception information.

Suggested by badkins.

pkgs/racket-doc/scribblings/guide/control.scrbl

@@ -117,6 +117,20 @@ predicate:
  (error "user break"))
 ]
 
+Exceptions carry information about the error that occurred. The
+@racket[exn-message] accessor provides a descriptive message for the
+exception. The @racket[exn-continuation-marks] accessor provides
+information about the point where the exception was raised.
+@margin-note{The @racket[continuation-mark-set->context] procedure provides best-effort structured backtrace information.}
+
+@interaction[
+(with-handlers ([exn:fail?
+                 (lambda (v)
+                   ((error-display-handler) (exn-message v) v))])
+  (car 17))
+]
+
+
 @; ----------------------------------------
 
 @section[#:tag "prompt"]{Prompts and Aborts}