net/url: document the package better

Changes made:
* Adjust the documented form for a URL to make it more obvious what
happens when the scheme is missing.
* Remove references to Go1.5. We are sufficiently far along enough
that this distinction no longer matters.
* Remove the "Opaque" example which provides a hacky and misleading
use of the Opaque field. This workaround is no longer necessary
since RawPath was added in Go1.5 and the obvious approach just works:
	// The raw string "/%2f/" will be sent as expected.
	req, _ := http.NewRequest("GET", "https://example.com/%2f/")

Fixes #18824

Change-Id: Ie33d27222e06025ce8025f8a0f04b601aaee1513
Reviewed-on: https://go-review.googlesource.com/36127
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/url/example_test.go

@@ -8,8 +8,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
-	"net/http"
-	"net/http/httputil"
 	"net/url"
 	"strings"
 )
@@ -59,33 +57,6 @@ func ExampleURL_roundtrip() {
 	// https://example.com/foo%2fbar
 }
 
-func ExampleURL_opaque() {
-	// Sending a literal '%' in an HTTP request's Path
-	req := &http.Request{
-		Method: "GET",
-		Host:   "example.com", // takes precedence over URL.Host
-		URL: &url.URL{
-			Host:   "ignored",
-			Scheme: "https",
-			Opaque: "/%2f/",
-		},
-		Header: http.Header{
-			"User-Agent": {"godoc-example/0.1"},
-		},
-	}
-	out, err := httputil.DumpRequestOut(req, true)
-	if err != nil {
-		log.Fatal(err)
-	}
-	fmt.Println(strings.Replace(string(out), "\r", "", -1))
-	// Output:
-	// GET /%2f/ HTTP/1.1
-	// Host: example.com
-	// User-Agent: godoc-example/0.1
-	// Accept-Encoding: gzip
-	//
-}
-
 func ExampleURL_ResolveReference() {
 	u, err := url.Parse("../../..//search?q=dotnet")
 	if err != nil {