Add array example for openapi_context

[reggermont]

I was facing this need and can't find the answer on the documentation. At first thought, I was trying to do something like example: "["/assets/img/photo1.jpg", "/assets/img/photo2.jpg"]" but it was returning errors or displaying the full string, as example is expecting a PlainText value

Had to try different methods to finally find my answer.

I think it should be in the documentation

Also fixed some old swagger_context declarations, as the last line says

To pass a context to the OpenAPI **v3** generator, use the `openapi_context` attribute (notice the prefix: `openapi_` instead of `swagger_`).

[vincentchalamon]

Please target 2.6 branch and rebase from it