docs: clarify in OpenAPI spec that at least one collection parameter is required for search

[kad-korpem]

Description

Clarify in the OpenAPI spec that the /search endpoint requires both the q parameter and at least one collection parameter (e.g., mycollection[version]=1).

Type of change

• Minor change (typo, formatting, version bump)
• Improvement of existing feature

Checklist:

• I've double-checked the code in this PR myself
• I've left the code better than before (boy scout rule)
• The code is readable, comments are added that explain hard or non-obvious parts.
• I've expanded/improved the (unit) tests, when applicable
• I've run (unit) tests that prove my solution works
• There's no sensitive information like credentials in my PR