explain what a box is in the reference, and add cross references to the guide

svn: r10753

collects/scribblings/guide/pairs.scrbl

@@ -3,7 +3,7 @@
           scribble/eval
           "guide-utils.ss")
 
-@title{Pairs and Lists}
+@title[#:tag "pairs"]{Pairs and Lists}
 
 A @deftech{pair} joins two arbitrary values. The @scheme[cons]
 procedure constructs pairs, and the @scheme[car] and @scheme[cdr]

collects/scribblings/reference/data.scrbl

@@ -133,6 +133,11 @@ for each pair of keywords is the same as using
 @; ------------------------------------------------------------
 @section[#:tag "boxes"]{Boxes}
 
+@guideintro["boxes"]{boxes}
+
+A @deftech{box} is like a single-element vector, normally used as
+minimal mutable storage.
+
 @defproc[(box? [v any/c]) boolean?]{
 
 Returns @scheme[#t] if @scheme[v] is a box, @scheme[#f] otherwise.}

collects/scribblings/reference/hashes.scrbl

@@ -3,6 +3,8 @@
 
 @title[#:tag "hashtables"]{Hash Tables}
 
+@guideintro["hash-tables"]{hash tables}
+
 A @deftech{hash table} (or simply @deftech{hash}) maps each of its
 keys to a single value. For a given hash table, keys are equivalent
 via @scheme[equal?] or @scheme[eq?], and keys are retained either

collects/scribblings/reference/pairs.scrbl

@@ -44,6 +44,8 @@
 
 @title[#:tag "pairs"]{Pairs and Lists}
 
+@guideintro["pairs"]{pairs and lists}
+
 @local-table-of-contents[]
 
 A @deftech{pair} combines exactly two values. The first value is

collects/scribblings/reference/vectors.scrbl

@@ -3,6 +3,8 @@
 
 @title[#:tag "vectors"]{Vectors}
 
+@guideintro["vectors"]{vectors}
+
 A @deftech{vector} is a fixed-length array with constant-time access
 and update of the vector slots, which are numbered from @scheme[0] to
 one less than the number of slots in the vector.