Collection::createIndex docs should direct users to server manual for index options

The API reference for Collection::createIndex() lists some index options, but has not been updated for new options since introduced in the server. Rather than try to keep this in sync, I think it'd be preferable to reduce the documentation to essential options and clearly point users to createIndexes in the server manual for a canonical list of supported options.

This came up in a code review for PHPLIB-856, since we do not document collection options for Database::modifyCollection.