docs(logs): stringify feature_flags example in Python best practices

The "Include Business Context" example showed feature_flags as a raw
list, which the Python SDK drops on ingestion (arrays aren't stored as
attributes). Show the join-to-string pattern instead so the example
actually works.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: sergical

platform-includes/logs/best-practices/python.mdx

@@ -69,7 +69,8 @@ sentry_sdk.logger.info(
 
         # Business context
         "order_value": 149.99,
-        "feature_flags": ["new-checkout", "discount-v2"],
+        # Join lists into a string — array values aren't stored as attributes
+        "feature_flags": ",".join(["new-checkout", "discount-v2"]),
     }
 )
 ```