After a discussion with @spicyj we decided to keep the warning even though the functions will throw in production. The warning is the only way to learn about that.

We should also document how to pass the secret through in case you know what you're doing.

*
 *       // A prop named "dialog" that requires an instance of Dialog.
 *       dialog: Props.instanceOf(Dialog).isRequired
 *     },
 *     render: function() { ... }
 *   });
 *
 * A more formal specification of how these methods are used:
 *
 *   type := array|bool|func|object|number|string|oneOf([...])|instanceOf(...)
 *   decl := ReactPropTypes.{type}(.isRequired)?
 *
 * Each and every declaration produces a function with the same signature. This
 * allows the creation of custom validation functions. For example:
 *
 *  var MyLink = React.createClass({
 *    propTypes: {
 *      // An optional string or URI prop named "href".
 *      href: function(props, propName, componentName) {
 *        var propValue = props[propName];
 *        if (propValue != null && typeof propValue !== 'string' &&
 *            !(propValue instanceof URI)) {
 *          return new Error(
 *            'Expected a string or an URI for ' + propName + ' in ' +
 *            componentName
 *          );
 *        }
 *      }
 *    },
 *    render: function() {...}