Analyze if query params can be used multiple times and if OAS documents it in the correct way

[annashamray]

Nitpicky, but ideally explode: true would be specified here to indicate that it should be used like ?data_attr=...&data_attr=... (see https://swagger.io/docs/specification/v3_0/serialization/#query-parameters)

For example: https://github.com/open-formulieren/open-forms/blob/3a702080d35c0670588cc69b8e49445e6224989c/src/openforms/appointments/api/views.py#L64

Although looking at it, apparently explode: true is considered the default by OAS3, so I'm not sure if our schema is correct in that case 🤔. Might be something to investigate in another issue

Originally posted by @stevenbal in #488 (comment)