net/http: document Server's implicit Content-Length response header

Fixes #23450

Change-Id: I829399194299d2e6d5e754b60e8f72b321b5da90
Reviewed-on: https://go-review.googlesource.com/115040Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/server.go

@@ -107,7 +107,7 @@ type ResponseWriter interface {
 	// is to prefix the Header map keys with the TrailerPrefix
 	// constant value. See TrailerPrefix.
 	//
-	// To suppress implicit response headers (such as "Date"), set
+	// To suppress automatic response headers (such as "Date"), set
 	// their value to nil.
 	Header() Header
 
@@ -117,7 +117,9 @@ type ResponseWriter interface {
 	// WriteHeader(http.StatusOK) before writing the data. If the Header
 	// does not contain a Content-Type line, Write adds a Content-Type set
 	// to the result of passing the initial 512 bytes of written data to
-	// DetectContentType.
+	// DetectContentType. Additionally, if the total size of all written
+	// data is under a few KB and there are no Flush calls, the
+	// Content-Length header is added automatically.
 	//
 	// Depending on the HTTP protocol version and the client, calling
 	// Write or WriteHeader may prevent future reads on the