reference: add pointers to immutable?

When reading just the vector section of the Reference, it's
not obvious that `immutable?` distinguishes immutable vectors,
so add a note there. And the same for sttrings, etc. The section
on boxes was missing the usual paragraph on mutability.

Based on a suggestion from Shriram.

pkgs/racket-doc/scribblings/reference/bytes.scrbl

@@ -15,6 +15,8 @@ string is provided to a procedure like @racket[bytes-set!], the
 @exnraise[exn:fail:contract]. Byte-string constants generated by the
 default reader (see @secref["parse-string"]) are immutable,
 and they are @tech{interned} in @racket[read-syntax] mode.
+Use @racket[immutable?] to check whether a byte string is
+immutable.
 
 Two byte strings are @racket[equal?] when they have the same length
 and contain the same sequence of bytes.

pkgs/racket-doc/scribblings/reference/data.scrbl

@@ -90,6 +90,14 @@ for each pair of keywords is the same as using
 A @deftech{box} is like a single-element vector, normally used as
 minimal mutable storage.
 
+A box can be @defterm{mutable} or
+@defterm{immutable}. When an immutable box is provided to a
+procedure like @racket[set-box!], the
+@exnraise[exn:fail:contract]. Box constants generated by the
+default reader (see @secref["parse-string"]) are
+immutable. Use @racket[immutable?] to check whether a box is
+immutable.
+
 A literal or printed box starts with @litchar{#&}. @see-read-print["box"]{boxes}
 
 @defproc[(box? [v any/c]) boolean?]{

pkgs/racket-doc/scribblings/reference/hashes.scrbl

@@ -25,7 +25,8 @@ table is also either mutable or immutable. Immutable hash tables
 support effectively constant-time access and update, just like mutable
 hash tables; the constant on immutable operations is usually larger,
 but the functional nature of immutable hash tables can pay off in
-certain algorithms.
+certain algorithms. Use @racket[immutable?] to check whether a hash
+table is immutable.
 
 @margin-note{Immutable hash tables actually provide @math{O(log N)}
 access and update. Since @math{N} is limited by the address space so

pkgs/racket-doc/scribblings/reference/strings.scrbl

@@ -14,6 +14,7 @@ procedure like @racket[string-set!], the
 @exnraise[exn:fail:contract]. String constants generated by the
 default reader (see @secref["parse-string"]) are
 immutable, and they are @tech{interned} in @racket[read-syntax] mode.
+Use @racket[immutable?] to check whether a string is immutable.
 
 Two strings are @racket[equal?] when they have the same length and
 contain the same sequence of characters.

pkgs/racket-doc/scribblings/reference/vectors.scrbl

@@ -16,7 +16,8 @@ the values in corresponding slots of the vectors are
 A vector can be @defterm{mutable} or @defterm{immutable}. When an
 immutable vector is provided to a procedure like @racket[vector-set!],
 the @exnraise[exn:fail:contract]. Vectors generated by the default
-reader (see @secref["parse-string"]) are immutable.
+reader (see @secref["parse-string"]) are immutable. Use
+@racket[immutable?] to check whether a vector is immutable.
 
 A vector can be used as a single-valued sequence (see
 @secref["sequences"]). The elements of the vector serve as elements