ot-shim: document the omission of scope manager

[dyladan]

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There was an issue in opentracing to add a scope manager, but it doesn't look like it was ever implemented https://github.com/opentracing/opentracing-javascript/issues/112

What is our action item here? Should we just document on the shim that there is no scope manager so we have no shim for it?

Originally posted by @dyladan in https://github.com/open-telemetry/opentelemetry-specification/issues/2219#issuecomment-1004811735