That's b/c it's a private class used internally by the ColumnModel. If you look at the source, you'll see it's nothing but an extension of Editor that defaults some configs.
customEditors : Object
An object containing name/value pairs of custom editor type definitions that allow the grid to support additional typ...
An object containing name/value pairs of custom editor type definitions that allow the grid to support additional types of editable fields. By default, the grid supports strongly-typed editing of strings, dates, numbers and booleans using built-in form editors, but any custom type can be supported and associated with a custom input control by specifying a custom editor. The name of the editor type should correspond with the name of the property that will use the editor. Example usage:
var grid = new Ext.grid.PropertyGrid({
...
customEditors: {
'Start Time': new Ext.grid.GridEditor(new Ext.form.TimeField({selectOnFocus:true}))
},
source: {
'Start Time': '10:00 AM'
}
});