docs: mention that relative bases are not supported

closes #5029

docs/en/reference/site-config.md

@@ -333,7 +333,7 @@ export default {
 - Type: `string`
 - Default: `/`
 
-The base URL the site will be deployed at. You will need to set this if you plan to deploy your site under a sub path, for example, GitHub pages. If you plan to deploy your site to `https://foo.github.io/bar/`, then you should set base to `'/bar/'`. It should always start and end with a slash.
+The base URL the site will be deployed at. You will need to set this if you plan to deploy your site under a sub path, for example, GitHub pages. If you plan to deploy your site to `https://foo.github.io/bar/`, then you should set base to `'/bar/'`. It should always start and end with a slash. Relative bases are not supported.
 
 The base is automatically prepended to all the URLs that start with / in other options, so you only need to specify it once.