Fix misleading gRPC connection params defaults

[olegbespalov]

What?

This change fixes the misleading "Defaults" for the grpc's connection params.

• MaxCallRecvMsgSize https://github.com/grafana/k6/blob/master/vendor/google.golang.org/grpc/rpc_util.go#L334-L336
• MaxCallSendMsgSize https://github.com/grafana/k6/blob/master/vendor/google.golang.org/grpc/rpc_util.go#L358-L360

Checklist

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• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the make docs command locally and verified that the changes look good.

Select one of these and delete the others:

If updating the documentation for the most recent release of k6:

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If updating the documentation for the next release of k6:

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Related PR(s)/Issue(s)

Closes #1434

[codebien]

LGTM. I agree with you we should move to a more human-friendly text.