raco setup: fix problem with doc index database

When a tag is serializable but not `write'--`read' invariant,
then it needs to be serialized and deserialized.

Also, clarify and check in `tag?' that a tag should be
serializable.

original commit: 6eef00a31287fd88e1c93139dc5b6c1ab97f6da8

collects/scribble/core.rkt

@@ -93,7 +93,8 @@
        (or (string? (cadr s))
            (generated-tag? (cadr s))
            (and (pair? (cadr s))
-                (list? (cadr s))))
+                (list? (cadr s))
+                (serializable? (cadr s))))
        (null? (cddr s))))
 
 (provide block?)

collects/scribblings/scribble/core.scrbl

@@ -3,6 +3,7 @@
           (except-in "utils.rkt" url)
           "struct-hierarchy.rkt" 
           (for-label scribble/manual-struct
+                     racket/serialize
                      file/convertible
                      setup/main-collects
                      scriblib/render-cond
@@ -1157,8 +1158,8 @@ or @racket[style] structure.}
 
 Returns @racket[#t] if @racket[v] is acceptable as a link
 @techlink{tag}, which is a list containing a symbol and either a
-string, a @racket[generated-tag] instance, or a list (of arbitrary
-values).}
+string, a @racket[generated-tag] instance, or a non-empty list
+of @racket[serializable?] values.}
 
 
 @defstruct[generated-tag ()]{