Fix reader error message when encountering illegal graph notation

Also, document explicitly that graph notation is not allowed at all in
read-syntax mode.

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

@@ -845,7 +845,8 @@ as constructed by @racket[pregexp], @litchar{#rx#} as constructed by
 
 A @graph-defn[] tags the following datum for reference via
 @graph-ref[], which allows the reader to produce a datum that
-has graph structure.
+has graph structure. Neither form is allowed in
+@racket[read-syntax] mode.
 
 For a specific @graph-tag[] in a single read result, each @graph-ref[]
 reference is replaced by the datum read for the corresponding

racket/src/expander/read/graph.rkt

@@ -57,8 +57,8 @@
                         "`#...~a` forms not ~a"
                         c
                         (if (read-config-for-syntax? config)
-                            "enabled"
-                            "allowed in `read-syntax` mode")))
+                            "allowed in `read-syntax` mode"
+                            "enabled")))
         (unless ((accum-string-count accum-str) . <= . 8)
           (reader-error in config
                         "graph ID too long in `~a~a~a`"

racket/src/racket/src/startup.inc

@@ -52913,8 +52913,8 @@ static const char *startup_source =
 "((c40_0) c_0)"
 "((temp41_0)"
 "(if(read-config-for-syntax? config_0)"
-"                                                            \"enabled\""
-"                                                            \"allowed in `read-syntax` mode\")))"
+"                                                            \"allowed in `read-syntax` mode\""
+"                                                            \"enabled\")))"
 "(reader-error12.1"
 " unsafe-undefined"
 " '#\\x"