improve changelog

Author: pront

changelog.d/multiline_interpolation_change.breaking.md

@@ -1,4 +1,20 @@
 The env var (and secrets) resolution now happens after the config string is parsed into a TOML table.
 As a side effect, this fixes a bug where comment lines referring to env vars (or secrets) that don't exist caused a config build error.
 
+This change breaks existing behavior. Injecting whole blocks now results in error e.g.
+
+```shell
+export SOURCES_BLOCK="sources:\"
+  demo:
+    type: demo_logs
+    format: json
+    interval: 1
+```
+
+```yaml
+${SOURCES_BLOCK}
+```
+
+The config above will fail to load.
+
 authors: pront