net/url: document, test that PathEscape escapes / to %2F

I couldn't remember and couldn't tell from the docs,
so I added a test and documented what I found.

Change-Id: Ic5d837c2d620b15d7a831823e94e70080f5e5324
Reviewed-on: https://go-review.googlesource.com/c/go/+/173948
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/url/url.go

@@ -276,8 +276,8 @@ func QueryEscape(s string) string {
 	return escape(s, encodeQueryComponent)
 }
 
-// PathEscape escapes the string so it can be safely placed
-// inside a URL path segment.
+// PathEscape escapes the string so it can be safely placed inside a URL path segment,
+// replacing special characters (including /) with %XX sequences as needed.
 func PathEscape(s string) string {
 	return escape(s, encodePathSegment)
 }

src/net/url/url_test.go

@@ -929,6 +929,11 @@ var pathEscapeTests = []EscapeTest{
 		"abc+def",
 		nil,
 	},
+	{
+		"a/b",
+		"a%2Fb",
+		nil,
+	},
 	{
 		"one two",
 		"one%20two",