net/http: expand documentation of Server.MaxHeaderBytes

Clarify that it includes the RFC 7230 "request-line".

Fixes #15494

Change-Id: I9cc5dd5f2d85ebf903229539208cec4da5c38d04
Reviewed-on: https://go-review.googlesource.com/22656Reviewed-by: Andrew Gerrand <adg@golang.org>

src/net/http/server.go

@@ -2035,9 +2035,15 @@ type Server struct {
 	Handler      Handler       // handler to invoke, http.DefaultServeMux if nil
 	ReadTimeout  time.Duration // maximum duration before timing out read of the request
 	WriteTimeout time.Duration // maximum duration before timing out write of the response
-	MaxHeaderBytes int           // maximum size of request headers, DefaultMaxHeaderBytes if 0
 	TLSConfig    *tls.Config   // optional TLS config, used by ListenAndServeTLS
 
+	// MaxHeaderBytes controls the maximum number of bytes the
+	// server will read parsing the request header's keys and
+	// values, including the request line. It does not limit the
+	// size of the request body.
+	// If zero, DefaultMaxHeaderBytes is used.
+	MaxHeaderBytes int
+
 	// TLSNextProto optionally specifies a function to take over
 	// ownership of the provided TLS connection when an NPN
 	// protocol upgrade has occurred. The map key is the protocol