docs: Add changelog entries covering spans PR

Author: alerque

fluent-bundle/CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
   - Implement NUMBER builtin
+  - Adapt for new optional source positions span feature in syntax parser
   - Improve examples
   - Refactor to remove unnecessary named lifetimes
   - Cleanup docs

fluent-syntax/CHANGELOG.md

@@ -1,6 +1,7 @@
 # Changelog
 
 ## Unreleased
+  - Add `spans` feature to track source positions in the AST
   - Add module `serializer`
   - De-ambiguate dependencies vs. features
   - Cleanup docs