document `define-runtime-path' limitation

Also, log a warning when it is used in a position where it
doesn't work rght with the executable creator. I didn't make
this case an error, because `define-runtime-path' can still
work in that case as long as no standalone executable needs
to be created.

collects/racket/runtime-path.rkt

@@ -158,6 +158,12 @@
                       #'orig-stx
                       id)))
                  ids)
+       (let ([phase (sub1 (variable-reference->phase (#%variable-reference)))])
+         (unless (zero? phase)
+           (log-warning "warning: ~a: at phase level ~a: ~e"
+                        (syntax-e (syntax-case #'orig-stx () [(id . _) #'id]))
+                        phase
+                        #'orig-stx)))
        #`(begin
            (define-values (id ...)
              (let-values ([(id ...) expr])

collects/scribblings/reference/filesystem.scrbl

@@ -536,6 +536,15 @@ If a path is needed only on some platforms and not on others, use
 @racket[define-runtime-path-list] with an @racket[expr] that produces an
 empty list on platforms where the path is not needed.
 
+Beware that @racket[define-runtime-path] in a @tech{phase level} other
+than 0 does not cooperate properly with an executable creator. To work
+around that limitation, put @racket[define-runtime-path] in a separate
+module---perhaps a @tech{submodule} created by @racket[module]---then
+export the definition, and then the module containing the definition
+can be @racket[require]d into any phase level. Using
+@racket[define-runtime-path] in a @tech{phase level} other than 0
+logs a warning at expansion time.
+
 The enclosing path for a @racket[define-runtime-path] is determined as
 follows from the @racket[define-runtime-path] syntactic form: