[docs] Extend the 'RequestMock: CORS validation failed for a request specified as <filter rule>' warning

[miherlosev]

At present:

Need to add a phrase how to fix this warning.
For example such a phrase:
To fix this warning specify the necessary CORS headers in respond method. For more detail see https://devexpress.github.io/testcafe/documentation/reference/test-api/requestmock/respond.html URL.

[lonely-caat]

@miherlosev what are actually the right headers to use when this error pops up?

[VasilyStrelyaev]

The server response should have the Access-Control-Allow-Origin header that indicates that cross-origin access is permitted. You can find other headers that can be included in the server's response in the HTTP response headers section.