net/url: document that Query returns only valid values

Fixes #19110

Change-Id: I291fa4ec3c61145162acd019e3f0e5dd3d7c97e9
Reviewed-on: https://go-review.googlesource.com/37194Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/url/url.go

@@ -974,6 +974,8 @@ func (u *URL) ResolveReference(ref *URL) *URL {
 }
 
 // Query parses RawQuery and returns the corresponding values.
+// It silently discards malformed value pairs.
+// To check errors use ParseQuery.
 func (u *URL) Query() Values {
 	v, _ := ParseQuery(u.RawQuery)
 	return v