raco setup: constrain Scribbled document names

Contrain the fourth element of a list describing a document in
`scribblings` (in "info.rkt") to a collection-path element, instead of
allowing an arbitrary relative path. This constraint is needed by
various parts of the build system, which assume (for example) that all
documents are rendered to an immediate subdirectory.

pkgs/racket-pkgs/racket-doc/scribblings/raco/setup.scrbl

@@ -287,16 +287,7 @@ Optional @filepath{info.rkt} fields trigger additional actions by
    A list of documents to build. Each document in the list is itself
    represented as a list, where each document's list starts with a
    string that is a collection-relative path to the document's source
-   file. A directory for pre-rendered documentation is computed from the
+   file.
-   source file name by starting with the directory of the @filepath{info.rkt}
-   file, adding @filepath{doc}, and then using the source file's name without
-   a suffix; if such a directory exists and does not have a
-   @filepath{synced.rktd} file, then it is treated as pre-rendered
-   documentation and moved into place, in which case the documentation source file
-   need not be present. (Moving documentation into place may require no movement
-   at all, depending on the way that the enclosing collection is installed, but 
-   movement includes adding a @filepath{synced.rktd} file to represent
-   the installation.)
 
    More precisely a @racketidfont{scribblings} entry must be a value
    that can be generated from an expression matching the following
@@ -326,7 +317,10 @@ Optional @filepath{info.rkt} fields trigger additional actions by
    (described further below). If a document's list contains a fourth
    item, @racket[_name], it is a name to use for the generated
    documentation, instead of defaulting to the source file's name
-   (sans extension), where @racket[#f] means to use the default. If a
+   (sans extension), where @racket[#f] means to use the default; a
+   non-@racket[#f] value for @racket[_name] must fit the grammar
+   of a colelction-name element as checked by 
+   @racket[collection-name-element?]. If a
    document's list contains a fifth item, @racket[_out-k], it is used
    a hint for the number of files to use for the document's
    cross-reference information; see below. If a document's list
@@ -467,7 +461,19 @@ Optional @filepath{info.rkt} fields trigger additional actions by
    less disables running the document in parallel to other documents.
    The main Racket reference is given a value of @racket[-11], the
    search page is given a value of @racket[10], and the default is
-   @racket[0].}
+   @racket[0].
+
+   A directory for pre-rendered documentation is computed from the
+   source file name by starting with the directory of the
+   @filepath{info.rkt} file, adding @filepath{doc}, and then using the
+   document name (which is usually the source file's name without a
+   suffix); if such a directory exists and does not have a
+   @filepath{synced.rktd} file, then it is treated as pre-rendered
+   documentation and moved into place, in which case the documentation
+   source file need not be present. Moving documentation into place
+   may require no movement at all, depending on the way that the
+   enclosing collection is installed, but movement includes adding a
+   @filepath{synced.rktd} file to represent the installation.}
 
  @item{@as-index{@racketidfont{release-note-files}} : @racket[(listof (cons/c string? (cons/c string? list?)))] ---
    A list of release-notes text files to link from the main documentation pages.

pkgs/racket-pkgs/racket-index/setup/scribble.rkt

@@ -5,6 +5,7 @@
          setup/path-to-relative
          "private/doc-path.rkt"
          setup/collects
+         setup/collection-name
          setup/main-doc
          setup/parallel-do
          setup/doc-db
@@ -154,7 +155,7 @@
     (define (validate path [flags '()] [cat '(library)] [name #f] [out-count 1] [order-hint 0])
       (and (string? path) (relative-path? path)
            (list? flags) (andmap scribblings-flag? flags)
-           (or (not name) (and (path-string? name) (relative-path? name) name))
+           (or (not name) (collection-name-element? name))
            (and (list? cat)
                 (<= 1 (length cat) 2)
                 (symbol? (car cat))