Enhance note about Java Bean getters and setters

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

@@ -529,9 +529,19 @@ the configuration of your application. For example:
 	}
 ----
 
-NOTE: The getters and setters are mandatory, since binding is via
+NOTE: The getters and setters are advisable, since binding is via
 standard Java Beans property descriptors, just like in Spring
-MVC. Some people use Project Lombok to add them automatically.
+MVC. They are mandatory for immutable types or those that are directly
+coercable from `String`. As long as they are initialized maps and
+collections need a getter but not necessarily a setter, for instance,
+since they can be mutated by the binder. Maps and collections can also
+be created (if there is a setter) and expanded (in any case), but
+arrays can not (so normally arrays need to be bound to a
+comma-separated value, but lists can be bound to individual
+elements). Nested POJO properties can also be created (so a setter is
+not mandatory) if they have a default constructor, or a constructor
+accepting a single value that can be coerced from String. Some people
+use Project Lombok to add getters and setters automatically.
 
 When the `@EnableConfigurationProperties` annotation is applied to your `@Configuration`,
 any beans annotated with `@ConfigurationProperties` will be automatically configured