Add retroactive release note for dbLoadTemplate in 7.0.9

This change was implemented in 1cd141c54 (dbLoadTemplate: error
propagation and more context on .db syntax error, 2022-02-24), but its
effects weren't documented then.

documentation/RELEASE-7.0.9.md

@@ -114,6 +114,12 @@ already be conditionally casting to/from the appropriate type.
 This release fixes the false positives failures when building with `_FORTIFY_SOURCE` level 3.
 The override introduced in 7.0.8.1 has been removed.
 
+### Improve error propagation in `dbLoadTemplate`
+
+If an error occurs while processing a `.substitutions` file (e.g. a syntax
+error in a `.template` file), `dbLoadTemplate` will stop processing that file
+and return an error. This means that further substitutions won't occur.
+
 ### Other
 
 - genVersionHeader: work with git submodules and worktrees.