Add Guide pointers to Refs on exceptions & control

collects/scribblings/guide/control.scrbl

@@ -176,7 +176,7 @@ section.
 
 @; ----------------------------------------------------------------------
 
-@section{Continuations}
+@section[#:tag "conts"]{Continuations}
 
 
 A @deftech{continuation} is a value that encapsulates a piece of an

collects/scribblings/reference/cont.scrbl

@@ -3,6 +3,8 @@
 
 @title[#:tag "cont"]{Continuations}
 
+@guideintro["conts"]{continuations}
+
 See @secref["cont-model"] and @secref["prompt-model"] for general
 information about continuations. Racket's support for prompts and
 composable continuations most closely resembles Dorai Sitaram's

collects/scribblings/reference/exns.scrbl

@@ -3,6 +3,8 @@
 
 @title[#:tag "exns"]{Exceptions}
 
+@guideintro["exns"]{exceptions}
+
 See @secref["exn-model"] for information on the Racket exception
 model. It is based on a proposal by Friedman, Haynes, and Dybvig
 @cite["Friedman95"].