Document that @distributed_trace is for internal usage within Azure SDKs

[lmolkova]

The decorator implements Azure SDK-specific features (e.g. suppression, support for certain sdks like opencensus), it should not be used by the end users in their applications.

We should document it as such and, if necessary, guide users to use similar generic OTel alternative

@tracer.start_as_current_span("name")
def function():
    ...

https://github.com/open-telemetry/opentelemetry-python/blob/a8aacb0c6f2f06bf19b501d98e62f7c0e667fa4c/opentelemetry-api/src/opentelemetry/trace/__init__.py#L386