sort & grouping properties not fully documented

[wesleycho]

The grouping property of columnDef is not documented at all - I'm not quite sure what are the options available here.

For the sort property for columnDef, the priority property is not documented, which appears to be necessary in order to have a default sort direction shown.

If some description of both are given, I'm more than willing to file a PR to improve the docs.

[PaulL1]

Grouping is documented here: http://ui-grid.info/docs/#/api/ui.grid.grouping.api:ColumnDef

The doco could stand to be updated for priority, although I think the tutorial shows it.

[wesleycho]

I found that grouping doc afterwards - I also saw the priority in the tutorial, but not in the API section (where I first looked).

What does the priority do though? I just set it to 0, but I'd need to understand what it does exactly in order to document it :) .

[lebolo]

There's a little documentation in http://ui-grid.info/docs/#/tutorial/102_sorting

You can set an initial sort state for the grid by defining the sort property on your column definitions. The direction sub-property says which way to sort, and the priority says what order to sort the columns in (lower priority gets sorted first).

Would be good to have it in the API section, mentioning that it starts counting from 0 =) Thx!

[JLLeitschuh]

Actually, that PR only solves half of the problem with the documentation. Ah well...