Polish documentation

Clarify that "spring.config.*" properties doesn't work in configuration
files.

Closes gh-4838

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

@@ -71,7 +71,7 @@ content into your application; rather pick only the properties that you need.
 	spring.cache.jcache.provider= # Fully qualified name of the CachingProvider implementation to use to retrieve the JSR-107 compliant cache manager. Only needed if more than one JSR-107 implementation is available on the classpath.
 	spring.cache.type= # Cache type, auto-detected according to the environment by default.
 
-	# SPRING CONFIG ({sc-spring-boot}/context/config/ConfigFileApplicationListener.{sc-ext}[ConfigFileApplicationListener])
+	# SPRING CONFIG - using environment property only ({sc-spring-boot}/context/config/ConfigFileApplicationListener.{sc-ext}[ConfigFileApplicationListener])
 	spring.config.location= # Config file locations.
 	spring.config.name=application # Config file name.
 

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

@@ -449,6 +449,10 @@ or
 	$ java -jar myproject.jar --spring.config.location=classpath:/default.properties,classpath:/override.properties
 ----
 
+WARNING: `spring.config.name` and `spring.config.location` are used very early to
+determine which files have to be loaded so they have to be defined as an environment
+property (typically OS env, system property or command line argument).
+
 If `spring.config.location` contains directories (as opposed to files) they should end
 in `/` (and will be appended with the names generated from `spring.config.name` before
 being loaded). The default search path `classpath:,classpath:/config,file:,file:config/`