[ci skip] Fix PiglinBarterEvent JavaDoc

[tal5]

Currently PiglinBarterEvent is documented as can be triggered by a piglin picking up an item that's on its bartering list., but it's fired when the barter is complete, right before the items are dropped. I went with the generic Called when X format most events use, let me know if it should include anything else.

[Machine-Maker]

I'd maybe keep the the first line of the javadoc there.

[tal5]

I'd maybe keep the the first line of the javadoc there.

Re-added