doc: deprecate order-of-magnitude

Already provided by `racket/math`

scribblings/gtp-util.scrbl

@@ -2,6 +2,8 @@
 
 @require[
   scribble/example
+  (for-syntax
+    racket/base)
   (for-label
     file/glob
     gtp-util/system
@@ -11,6 +13,7 @@
     racket/contract
     racket/path
     (only-in racket/math natural?)
+    (prefix-in rkt: (only-in racket/math order-of-magnitude))
     (only-in openssl/md5 md5))]
 
 @(define (make-gtp-util-eval) (make-base-eval '(require gtp-util)))
@@ -109,6 +112,9 @@ If you think one of these functions should "graduate" to another library
 
 @defproc[(order-of-magnitude [n real?]) natural?]{
   Count the number of digits in the given number.
+
+  @(let-syntax ([order-of-magnitude (make-rename-transformer #'rkt:order-of-magnitude)])
+     @deprecated[#:what "function" @racket[order-of-magnitude]]{(from @racketmodname[racket/math])})
 }
 
 @defproc[(file-remove-extension [ps path-string?]) path-string?]{