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 {

src/net/url/url.go

@@ -309,9 +309,10 @@ func escape(s string, mode encoding) string {
 }
 
 // A URL represents a parsed URL (technically, a URI reference).
+//
 // The general form represented is:
 //
-//	scheme://[userinfo@]host/path[?query][#fragment]
+//	[scheme:][//[userinfo@]host][/]path[?query][#fragment]
 //
 // URLs that do not start with a slash after the scheme are interpreted as:
 //
@@ -321,23 +322,16 @@ func escape(s string, mode encoding) string {
 // A consequence is that it is impossible to tell which slashes in the Path were
 // slashes in the raw URL and which were %2f. This distinction is rarely important,
 // but when it is, code must not use Path directly.
-//
-// Go 1.5 introduced the RawPath field to hold the encoded form of Path.
 // The Parse function sets both Path and RawPath in the URL it returns,
 // and URL's String method uses RawPath if it is a valid encoding of Path,
 // by calling the EscapedPath method.
-//
-// In earlier versions of Go, the more indirect workarounds were that an
-// HTTP server could consult req.RequestURI and an HTTP client could
-// construct a URL struct directly and set the Opaque field instead of Path.
-// These still work as well.
 type URL struct {
 	Scheme     string
 	Opaque     string    // encoded opaque data
 	User       *Userinfo // username and password information
 	Host       string    // host or host:port
-	Path       string
-	RawPath    string // encoded path hint (Go 1.5 and later only; see EscapedPath method)
+	Path       string    // path (relative paths may omit leading slash)
+	RawPath    string    // encoded path hint (see EscapedPath method)
 	ForceQuery bool      // append a query ('?') even if RawQuery is empty
 	RawQuery   string    // encoded query values, without '?'
 	Fragment   string    // fragment for references, without '#'
@@ -351,6 +345,7 @@ func User(username string) *Userinfo {
 
 // UserPassword returns a Userinfo containing the provided username
 // and password.
+//
 // This functionality should only be used with legacy web sites.
 // RFC 2396 warns that interpreting Userinfo this way
 // ``is NOT RECOMMENDED, because the passing of authentication