Enable alias "swap" with AliasParams

The existing ElasticClient.Swap only takes an alias name and as a result
any alias filters, routing, etc. are lost during the swap b/c "swap" is
actually a delete + add. This adds an override to Swap that takes an
AliasParams argument so that you can atomically swap more complicated
aliases.

I also updated the doc summary for Swap while I was in there.