Document need to avoid Logback placeholder format in logging properties

Closes gh-4731

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

@@ -1231,6 +1231,14 @@ To help with the customization some other properties are transferred from the Sp
 All the logging systems supported can consult System properties when parsing their
 configuration files.  See the default configurations in `spring-boot.jar` for examples.
 
+[TIP]
+====
+If you want to use a placeholder in a logging property, you should use
+<<boot-features-external-config-placeholders-in-properties,Spring Boot's syntax>> and not
+the syntax of the underlying framework. Notably, if you're using Logback, you should use
+`:` as the delimiter between a property name and its default value and not `:-`.
+====
+
 [TIP]
 ====