net/http/httptest: add more docs on ResponseRecord fields

Fixes #16717

Change-Id: I7b6518609796a537437539c35461a18e9e6f207f
Reviewed-on: https://go-review.googlesource.com/32190Reviewed-by: Martin Möhrmann <martisch@uos.de>

src/net/http/httptest/recorder.go

@@ -15,9 +15,25 @@ import (
 // ResponseRecorder is an implementation of http.ResponseWriter that
 // records its mutations for later inspection in tests.
 type ResponseRecorder struct {
-	Code      int           // the HTTP response code from WriteHeader
-	HeaderMap http.Header   // the HTTP response headers
-	Body      *bytes.Buffer // if non-nil, the bytes.Buffer to append written data to
+	// Code is the HTTP response code set by WriteHeader.
+	//
+	// Note that if a Handler never calls WriteHeader or Write,
+	// this might end up being 0, rather than the implicit
+	// http.StatusOK. To get the implicit value, use the Result
+	// method.
+	Code int
+
+	// HeaderMap contains the headers explicitly set by the Handler.
+	//
+	// To get the implicit headers set by the server (such as
+	// automatic Content-Type), use the Result method.
+	HeaderMap http.Header
+
+	// Body is the buffer that a Handler's Write calls are sent to.
+	// If nil, the Writes are silently discard.
+	Body *bytes.Buffer
+
+	// Flushed is whether the Handler called Flush.
 	Flushed bool
 
 	result      *http.Response // cache of Result's return value