[DOCS] Update intro to Search APIs > Request Body Search > Preference

[gchaps]

The Preference page has an intro paragraph with this sentence:

By default, the operation is randomized among the available shard copies.

The should be changed to

By default, the operation is randomized among the available shard copies, unless allocation awareness is used.

This reason for this change is discussed in Kibana issue 17271.

[elasticmachine]

Pinging @elastic/es-search-aggs