[DOC] Add documentation for new preferences for search requests

[kotwanikunal]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.
As a part of opensearch-project/OpenSearch#7375, we added a new options for preferences to search requests - _primary, _replica, _primary_first and _replica_first.

None of the preferences are listed out on https://opensearch.org/docs/2.7/api-reference/search/ (Request parameters -> Preferences) but it would be useful for the clients and users to have a list of available preferences.

_primary - Search will be performed only on the primary shards.
_replica - Search will be performed only on the replica shards. 
_primary_first - Search will be performed preferably on the primary shards, but can failover to other available shards. 
_replica_first - Search will be performed preferably on the replica shards, but can failover to other available shards including primary. 

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

• Add back primary shard preference for queries OpenSearch#7375
• [DISCUSS] Add back preference for searching _primaries or _replicas OpenSearch#6046