Default description for field instances in _metatags_quick_init_basic_fields()

Hi,

I would like to have default description text available to the field instances that get created for every bundle from the admin settings form. This makes it so much easier in the same step as creating fields to have helpful description text so that editors of my site can straight away understand what 'Keywords' and 'Descriptions' mean, as by themselves they are not very intuitive to deduce their context.

Hope the attached patch helps.