Fix detection of dot-based resource bundle basenames

This commit makes sure that properties-based resource bundle location
with a dot is detected. It also harmonizes the description of the
configuration key as our support is not stricly matching the convention.

Closes gh-10092

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -114,7 +114,7 @@ content into your application; rather pick only the properties that you need.
 
 	# INTERNATIONALIZATION ({sc-spring-boot-autoconfigure}/context/MessageSourceAutoConfiguration.{sc-ext}[MessageSourceAutoConfiguration])
 	spring.messages.always-use-message-format=false # Set whether to always apply the MessageFormat rules, parsing even messages without arguments.
-	spring.messages.basename=messages # Comma-separated list of basenames, each following the ResourceBundle convention.
+	spring.messages.basename=messages # Comma-separated list of basenames (essentially a fully-qualified classpath location), each following the ResourceBundle convention with relaxed support for slash based locations.
 	spring.messages.cache-seconds=-1 # Loaded resource bundle files cache expiration, in seconds. When set to -1, bundles are cached forever.
 	spring.messages.encoding=UTF-8 # Message bundles encoding.
 	spring.messages.fallback-to-system-locale=true # Set whether to fall back to the system Locale if no files for a specific Locale have been found.