Fix _source accepting an array of fields in _search.

[dblock]

Description

Coming from opensearch-project/opensearch-java#1087, _source accepts an array of fields in _search. Looks like we also don't properly document this in https://opensearch.org/docs/latest/api-reference/search.

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[github-actions]

Changes Analysis

Commit SHA: 49817b1
Comparing To SHA: 82c000f

API Changes

Summary

└─┬Components
  └─┬_core.search:SourceConfig
    ├──[➕] oneOf (38360:11)
    ├─┬ONEOF
    │ └──[🔀] $ref (38361:11)❌ 
    └─┬ONEOF
      └──[🔀] $ref (38373:13)❌ 

Document Element	Total Changes	Breaking Changes
components	3	2

• ❌ BREAKING Changes: 2 out of 3
• Modifications: 2
• Additions: 1
• Breaking Modifications: 2

Report

The full API changes report is available at: https://github.com/opensearch-project/opensearch-api-specification/actions/runs/9977843973/artifacts/1711603133

API Coverage

	Before	After	Δ
Covered (%)	483 (47.31 %)	483 (47.31 %)	0 (0 %)
Uncovered (%)	538 (52.69 %)	538 (52.69 %)	0 (0 %)
Unknown	24	24	0