diff --git a/changelog.d/19243.doc b/changelog.d/19243.doc
new file mode 100644
index 000000000..3a396c88d
--- /dev/null
+++ b/changelog.d/19243.doc
@@ -0,0 +1 @@
+Document in the `--config-path` help how multiple files are merged - by merging them shallowly.
diff --git a/synapse/config/_base.py b/synapse/config/_base.py
index 95a00c671..43dece4a0 100644
--- a/synapse/config/_base.py
+++ b/synapse/config/_base.py
@@ -672,7 +672,8 @@ class RootConfig:
             action="append",
             metavar="CONFIG_FILE",
             help="Specify config file. Can be given multiple times and"
-            " may specify directories containing *.yaml files.",
+            " may specify directories containing *.yaml files."
+            " Top-level keys in later files overwrite ones in earlier files.",
         )
         parser.add_argument(
             "--no-secrets-in-config",