docs: point to location branch of a config (#5732)

Co-authored-by: Rhys Arkins <rhys@arkins.net>

docs/usage/configuration-options.md

@@ -19,6 +19,8 @@ You can store your Renovate configuration file in one of the following locations
 - `.renovaterc`
 - `package.json` _(within a `"renovate"` section)_
 
+Renovate always uses the config from the repository's default branch, even if that configuration specifies multiple `baseBranches`. Renovate does not read/override the config from within each base branch if present.
+
 Also, be sure to check out Renovate's [shareable config presets](/config-presets/) to save yourself from reinventing any wheels.
 
 If you have any questions about the below config options, or would like to get help/feedback about a config, please post it as an issue in [renovatebot/config-help](https://github.com/renovatebot/config-help) where we will do our best to answer your question.