Say why the Digest uri is in origin form

Through a plain-HTTP proxy the request line carries the absolute form,
while the Digest uri stays in origin form, as curl sends it. The comment
cited RFC 7616 3.4, which asks for the effective request URI.

httplib.h

@@ -10888,7 +10888,8 @@ inline bool process_client_socket_ssl(
   return callback(strm);
 }
 
-// `uri` is req's request-target in origin form (RFC 7616 3.4).
+// `uri` is req's request-target in origin form, as curl sends it, also when
+// the request line carries the absolute form for a proxy.
 inline std::pair<std::string, std::string> make_digest_authentication_header(
     const Request &req, const std::string &uri,
     const std::map<std::string, std::string> &auth, size_t cnonce_count,