Clarify span_id attribute

Author: markushi

develop-docs/sdk/data-model/event-payloads/contexts.mdx

@@ -628,7 +628,7 @@ Additional information that allows Sentry to connect multiple transactions, span
 
 `span_id`
 
-: _Required_. The ID of the span.
+: _Required_. The ID of the span or transaction. If there's none available (e.g. in tracing without performance mode), a random span ID must be provided.
 
 - Example: `"bb8f278130535c3c"`