scribble/manual: add #:id' option to defthing'

Use this open in the HtDP language documentation in the same
way as for `defproc', which fixes the docs for `pi', `e', etc.

original commit: 8d03995b628771e529aa56403dbdd29a8e0926d5

collects/scribble/private/manual-proc.rkt

@@ -913,18 +913,21 @@
 
 ;; ----------------------------------------
 
-(define-syntax defthing
-  (syntax-rules ()
-    [(_ #:kind kind id result desc ...)
-     (with-togetherable-racket-variables
-      ()
-      ()
-      (*defthing kind
-                 (list (quote-syntax/loc id)) (list 'id) #f
-                 (list (racketblock0 result))
-                 (lambda () (list desc ...))))]
-    [(_ id result desc ...)
-     (defthing #:kind #f id result desc ...)]))
+(define-syntax (defthing stx)
+  (syntax-parse stx
+    [(_ kind:kind-kw 
+        (~optional (~seq #:id id-expr)
+                   #:defaults ([id-expr #'#f]))
+        id 
+        result 
+        desc ...)
+     #'(with-togetherable-racket-variables
+        ()
+        ()
+        (*defthing kind.kind
+                   (list (or id-expr (quote-syntax/loc id))) (list 'id) #f
+                   (list (racketblock0 result))
+                   (lambda () (list desc ...))))]))
 
 (define-syntax defthing* 
   (syntax-rules ()

collects/scribblings/scribble/manual.scrbl

@@ -1040,18 +1040,26 @@ Like @racket[defparam], but the contract on a parameter argument is
 @racket[boolean?].}
 
 
-@defform[(defthing maybe-kind id contract-expr-datum pre-flow ...)]{
+@defform/subs[(defthing maybe-kind maybe-id id contract-expr-datum
+                pre-flow ...)
+              ([maybe-kind code:blank
+                           (code:line #:kind kind-string-expr)]
+               [maybe-id code:blank
+                         (code:line #:id id-expr)])]{
 
 Like @racket[defproc], but for a non-procedure binding.
 
-If @racket[#:kind kind-string-expr] is supplied as
-@racket[maybe-kind], it is used in the same way as for
+If @racket[#:kind kind-string-expr] is supplied,
+it is used in the same way as for
 @racket[defproc], but the default kind is @racket["value"].
 
+If @racket[#:id id-expr] is supplied, then the result of
+@racket[id-expr] is used in place of @racket[id].
+
 Examples:
 @codeblock[#:keep-lang-line? #f]|{
 #lang scribble/manual
-@defthing[moldy-sandwich sandwich?]
+@defthing[moldy-sandwich sandwich?]{
   Don't eat this. Provided for backwards compatibility.
 }
 }|