Ajax: replace "jqXHR.complete" callback with "always"

Since it was deprecated since 1.8.

Also add additional comments which explains tricky
behaviour of "always" callback

Closes gh-2033

src/ajax/load.js

@@ -66,7 +66,10 @@ jQuery.fn.load = function( url, params, callback ) {
 				// Otherwise use the full result
 				responseText );
 
-		}).complete( callback && function( jqXHR, status ) {
+		// If the request succeeds, this function gets "data", "status", "jqXHR"
+		// but they are ignored because response was set above.
+		// If it fails, this function gets "jqXHR", "status", "error"
+		}).always( callback && function( jqXHR, status ) {
 			self.each( callback, response || [ jqXHR.responseText, status, jqXHR ] );
 		});
 	}