read/recursive: avoid placeholder wrapper

Although the documentation claimed that `read/recursive` produces
a plaeholder, that seems to be a leftover from a much older
reader (before `make-reader-graph`). Fix the new `read/recursive`
to be like the old one, and update the documentation.

Thanks to Alex Knauth for tracking down the unnecessary change
in reader behavior.

Related to #2099

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

@@ -62,17 +62,9 @@ When @racket[graph?] is @racket[#f], graph structure annotations in
 the read datum are local to the datum.
 
 When called within the dynamic extent of @racket[read], the
-@racket[read/recursive] procedure produces either an opaque
-placeholder value, a special-comment value, or an end-of-file.  The
-result is a special-comment value (see @secref["special-comments"])
-when the input stream's first non-whitespace content parses as a
-comment. The result is end-of-file when @racket[read/recursive]
-encounters an end-of-file. Otherwise, the result is a placeholder that
-protects graph references that are not yet resolved. When this
-placeholder is returned within an S-expression that is produced by any
-reader-extension procedure (see @secref["reader-procs"]) for the
-same outermost @racket[read], it will be replaced with the actual read
-value before the outermost @racket[read] returns.
+@racket[read/recursive] procedure can produce a special-comment value
+(see @secref["special-comments"]) when the input stream's first
+non-whitespace content parses as a comment.
 
 See @secref["readtables"] for an extended example that uses
 @racket[read/recursive].

pkgs/racket-test-core/tests/racket/read.rktl

@@ -1125,6 +1125,27 @@
 (test #\{ syntax-property (read-syntax 'x (open-input-string "{1 2 3}")) 'paren-shape)
 (test #\{ syntax-property (read-syntax 'x (open-input-string "#{1 2}")) 'paren-shape)
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Check `read/recursive`
+
+(let ()
+  (define in (open-input-string "abc"))
+  (test 'abc read/recursive in))
+
+(let ()
+  (define in2 (open-input-string "(#0=(abc) #z(#0#))"))
+  (parameterize ([current-readtable
+                  (make-readtable (current-readtable)
+                                  #\z
+                                  'dispatch-macro
+                                  (lambda (char in name line col pos)
+                                    (define v (read/recursive in))
+                                    (log-error ">> ~s" v)
+                                    v))])
+    (define v (read in2))
+    (test '((abc) ((abc))) values v)
+    (test #t eq? (car v) (caadr v))))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Test read error on a character not in any port
 

racket/src/expander/read/main.rkt

@@ -103,7 +103,7 @@
          (not (eof-object? v))
          (not (special-comment? v)))
     (get-graph-hash config) ; to trigger placeholder resolution
-    (make-placeholder v)]
+    v]
    [else v]))
 
 (define (read-language in fail-k