Unintuitive tax query 'operator' values

[scribu]

Currently, the accepted values for the tax_query 'operator' arg are:

'AND', 'IN' and 'NOT IN'.

It would be better if they were instead:

'AND', 'OR' and 'NOT'.

which would be more intuitive and consistent with wp_list_filter().

Obviously, we would still have to keep backwards compatibility with the old values.