update migration guide

Author: antonpirker

MIGRATION_GUIDE.md

@@ -9,6 +9,7 @@ Looking to upgrade from Sentry SDK 2.x to 3.x? Here's a comprehensive list of wh
 ### Changed
 
 - The SDK now supports Python 3.7 and higher.
+- The default of `traces_sample_rate` changed to `0`. Meaning: Incoming traces will be continued by default. For example, if your frontend sends a `sentry-trace/baggage` headers pair, your SDK will create Spans and send them to Sentry. (The default used to be `None` meaning by default no Spans where created, no matter what headers the frontend sent to your project.) See also: https://docs.sentry.io/platforms/python/configuration/options/#traces_sample_rate
 - `sentry_sdk.start_span` now only takes keyword arguments.
 - `sentry_sdk.start_transaction`/`sentry_sdk.start_span` no longer takes the following arguments: `span`, `parent_sampled`, `trace_id`, `span_id` or `parent_span_id`.
 - You can no longer change the sampled status of a span with `span.sampled = False` after starting it.

tests/tracing/test_trace_propagation.py

@@ -22,7 +22,7 @@
 #
 # Proper high level testing for trace propagation.
 # Testing the matrix of test cases described here:
-# https://docs.google.com/spreadsheets/d/1IyOTYIC2bwu6HeHrxbLHAm6Lq44atVzf2TDJoPCMDZA/edit?gid=0#gid=0
+# https://develop.sentry.dev/sdk/telemetry/traces/trace-propagation-cheat-sheet/
 #