Arbitrary preference for search.asciidoc

Arbitrary string

The preference parameter can be set to any arbitrary string, such as the session ID of a user, which would ensure that the user always gets results from the same node. While this is less useful when retrieving individual documents, it can be very useful when searching: two documents that are ranked as equally relevant by a search query may be returned in a different order by different shards. Always returning results from the same shard means that the user will see the results in a consistent order.