Add info about how to prefer local default version to Bazel documentation.
RELNOTES: None.
@@ -89,4 +89,5 @@ target containing the locally available Xcode versions. For `local_versions`,
you probably want to use the autogenerated
`@local_config_xcode//:host_available_xcodes`. The default Xcode version is the
newest mutually available version, if there is one, otherwise the default of the
-`local_versions` target.
+`local_versions` target. If you prefer to use the `local_versions` default
+as the default, you can pass `--experimental_prefer_mutual_default=false`.
