api.php auto-generates excellent API documentation. However, it omits the type of most parameters and the default for some. It shows the type if it has a limited namespace, range, or set of values, but it doesn't simply tell you "Type: string" or "Type: integer". Similarly, it often omits the default for a parameter.
For example, look at http://en.wikipedia.org/w/api.php for action=edit. This has a redirect parameter, but there's no indication of its type or what the default behavior is. The code specifies this information:
'redirect' => array( ApiBase::PARAM_TYPE => 'boolean', ApiBase::PARAM_DFLT => false, ),
but makeHelpMsgParameters() in ApiBase.php doesn't always output $type and $default.