net/http: document that Basic Auth may require URL encoding

Explicitly warn callers that no URL encoding is performed and
that they might need to do it.

Fixes #31577

Change-Id: I52dc3fd2798ba8c3652d4a967b1c5c48eb69f43b
Reviewed-on: https://go-review.googlesource.com/c/go/+/173319Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/request.go

@@ -912,6 +912,10 @@ func parseBasicAuth(auth string) (username, password string, ok bool) {
 //
 // With HTTP Basic Authentication the provided username and password
 // are not encrypted.
+//
+// Some protocols may impose additional requirements on pre-escaping the
+// username and password. For instance, when used with OAuth2, both arguments
+// must be URL encoded first with url.QueryEscape.
 func (r *Request) SetBasicAuth(username, password string) {
 	r.Header.Set("Authorization", "Basic "+basicAuth(username, password))
 }