Document DryRun mode will not work when EnableTraceLocality is set to true

[VinozzZ]

When trace locality is disabled, the system currently publishes drop decisions with only trace IDs. This approach does not provide enough information for peers to decorate spans with the original metadata generated during trace decisions, which is available when trace locality is enabled.

We have decided not to support dry-run mode in the upcoming changes related to removing trace locality.
We should make sure to document this when the feature is released