add comment explaining why the concept of http-reader-writer-body

(which is used by the seval-handler only) doesn't work

scheme/httpd/response.scm

@@ -46,6 +46,15 @@
   writer-body?
   (proc writer-body-proc))
 
+;; the concept of http-reader-writer-body doesn't work: status-line
+;; and headers of the response (i.e. the whole http-response record)
+;; have to be built _before_ we have seen the entity-body of the
+;; request. (Not until display-http-body hands over the iport to
+;; reader-writer-body the entity-body can be read in). If the
+;; entity-body is erroneous or if we encounter a server internal error
+;; while reading in the entity-body we are not able to send an
+;; appropriate response. (At that point of time we already sent
+;; status-line and response-headers!)
 (define-record-type http-reader-writer-body :http-reader-writer-body
   (make-reader-writer-body proc)
   reader-writer-body?

scheme/httpd/seval.scm

@@ -9,6 +9,10 @@
 ;;; This is really just an handler example demonstrating how to upload code 
 ;;; into the server.
 
+;;; Besides, this handler has always been broken because it makes use
+;;; of the concept of http-reader-writer-body which is broken
+;;; itself. See response.scm.
+
 ;;; (do/timeout secs thunk)
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;; Run THUNK, and gun it down if it hasn't finished in SECS seconds.
@@ -54,7 +58,7 @@
      (time)
      "text/html"
      '()
-     (make-reader-writer-body
+     (make-reader-writer-body ;; see response.scm for an explanation why the concept of http-reader-writer-body doesn't work
       (lambda (iport oport options)
 	(with-fatal-error-handler