it's not technically part of the public API; however, it actually overrides the xtype 'combo', so if you use:

{
xtype: 'combo',
....other config...
}

it will create one. you can see the code in FormPanel.js, which is part of the API.

The majority of the options I described above are standard Ext, and your best bet if the Ext doc for those. If you need multi-select (ie. LOVCombo), then using the LABKEY.ext.Combo really wouldnt apply here.