Clarify ref-database configuration location

Currently global-refdb can be consumed by (and configured by)
multiple modules, which can lead to confusion.

Change-Id: I01748dd1db3ad09175cbe33fd713723f1dc87d37

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 7d1159b..2423208 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -4,6 +4,13 @@
 The plugin configuration belongs in @PLUGIN@.config.
 The credentialsPath is required when not using the Spanner emulator.
 
+This plugin implements [global-refdb](https://gerrit.googlesource.com/modules/global-refdb/+/HEAD/config.md).
+Global-refdb configuration can be placed in the config file of any installed module
+that binds the ref-database implementation, for example, `high-availability.config`
+or `multi-site.config`, not only in `@PLUGIN@.config`. If ref-database settings do
+not appear here, check the config file of whichever extension is responsible for
+binding the ref-db in your deployment.
+
 ```
 [ref-database "spanner"]
         instance = spanner-instance