[DS] Document dynamic sampling questions and considerations

[AbhiPrasad]

#613 provides a initial foundation for the DS/baggage docs - but there were some outstanding TODOs still left to document. This GH issue provides a place for us to store that:

TODO:

• Consequences of immutable baggage and impacts it might have on DS UX
• Document challenge about 3rd party APIs that are instrumented with DS compatible SDKs
• Add note about sample rate smoothing (and how it uses sample_rate in trace contexts)
• Dynamic Sampling on traces vs. transactions

[Lms24]

One more item

• How to handle the edge case of receiving a baggage header with sentry- keyed entries on incoming requests but no sentry-trace header