Clarify that baseName in ResourceBundleMessageSource does not support multiple locations

[snicoll]

See spring-projects/spring-boot#26712 for background.

There are various places where multiple locations can be searched for and this one is delegating ultimately to ResourceBundle#getBundle. It would be nice if the reference guide was extra explicit about the fact that multiple bundles at the same location in the classpath in different modules won't be merged.