docs: Update MIGRATION.md

Author: JPeer264

MIGRATION.md

@@ -872,7 +872,7 @@ Attribute availability remains runtime-dependent. For example, browser and Worke
 
 Span attributes now use the shared `@sentry/conventions` package under the hood.
 The deprecated `semanticAttributes` re-export was removed. Import span attribute constants from `@sentry/core` directly.
-`SEMANTIC_ATTRIBUTE_SENTRY_SOURCE` (`sentry.source`) was removed. Use the `sentry.segment.name.source` attribute instead. The SDK does not export a constant for it, import `SENTRY_SEGMENT_NAME_SOURCE` from `@sentry/conventions/attributes` or use the string.
+`SEMANTIC_ATTRIBUTE_SENTRY_SOURCE` (`sentry.source`) was removed. Use the `sentry.segment.name.source` attribute instead and make sure to only set it on segment/root spans.
 `sentry.segment.name.source` is only set on the root span. Setting it on a child span is a no-op: `setAttribute` ignores it, and a value passed in a child span's initial attributes is dropped when the span is linked to its parent.
 
 ### Span operation (`op`) changes