Clarify "YAML Shortcomings" documentation

Closes gh-17933

Author: philwebb

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

@@ -691,22 +691,28 @@ YAML files cannot be loaded by using the `@PropertySource` annotation.
 So, in the case that you need to load values that way, you need to use a properties file.
 
 Using the multi YAML document syntax in profile-specific YAML files can lead to unexpected behavior.
-For example, consider the following config in a file called `application-dev.yml`, with the `dev` profile being active:
+For example, consider the following config in a file:
 
+.application-dev.yml
 [source,yaml,indent=0]
 ----
 	server:
 	  port: 8000
 	---
 	spring:
-	  profiles: !test
+	  profiles: "!test"
 	  security:
 	    user:
-	      password: weak
+	      password: "secret"
 ----
 
-In the example above, profile negation and profile expressions will not behave as expected.
-We recommend that you don't combine profile-specific YAML files and multiple YAML documents and stick to using only one of them.
+If you run the application with the arguments `--spring.profiles.active=dev" you might expect `security.user.password` to be set to "`secret`", but this is not the case.
+
+The nested document will be filtered because the main file is named `application-dev.yml`.
+It is already considered to be profile-specific, and nested documents will be ignored.
+
+TIP: We recommend that you don't mix profile-specific YAML files and multiple YAML documents.
+Stick to using only one of them.