Clarify scope attributes for zipkin and jaeger exporters

[jack-berg]

The OpenTelemetry Transformation to non-OTLP formats has the following to say about scope attributes:

Exporters to formats that don't have a concept that is equivalent to the Scope SHOULD record the attributes at the most suitable place in their corresponding format, typically at the Span, Metric or LogRecord equivalent.

The Zipkin and jaeger are in this category since they don't have the equivalent of scope attributes. However, they already have documents explicitly defining their respective transformations.

It seems like the general advice of recording scope attributes as record level attributes should apply to Zipkin and Jaeger, but it would be good to explicitly state this.

[pirgeo]

This also seems related to #2736. When moving attributes from the OTLP scope to the Span/Metric data point/Log record, there might be an overlap in attribute keys, and a clash-resolution strategy is required.