Make it clearer when there will be no auto-configured MessageSource

Closes gh-17485

Author: wilkinsona

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -2015,7 +2015,8 @@ a `messages` resource bundle at the root of the classpath.
 NOTE: The auto-configuration applies when the default properties file for the configured
 resource bundle is available (i.e. `messages.properties` by default). If your resource
 bundle contains only language-specific properties files, you are required to add the
-default.
+default. If no properties file is found that matches any of the configured base names,
+there will be no auto-configured `MessageSource`.
 
 The basename of the resource bundle as well as several other attributes can be configured
 using the `spring.messages` namespace, as shown in the following example: