Update skewed join documentation

While working on Tez skewed join, I found two problems with the skewed join documentation-

1. The following is misleading because skewed join also works with left, right, and full outer joins.

   Skewed join works with two-table inner join.

2. In two-way join, the 1st table must be the skewed one because Pig samples on that. But this is not mentioned anywhere.