Improve @value documentation

Improve "@ConfigurationProperties vs. @value" documentation with the
recommendation that kebab-case is used with the `@Value` annotation.

Closes gh-20507

Author: philwebb

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

@@ -1303,7 +1303,7 @@ The following table summarizes the features that are supported by `@Configuratio
 
 | <<boot-features-external-config-relaxed-binding,Relaxed binding>>
 | Yes
-| No
+| Limited (see note below)
 
 | <<appendix-configuration-metadata.adoc#configuration-metadata,Meta-data support>>
 | Yes
@@ -1315,7 +1315,12 @@ The following table summarizes the features that are supported by `@Configuratio
 |===
 
 If you define a set of configuration keys for your own components, we recommend you group them in a POJO annotated with `@ConfigurationProperties`.
-You should also be aware that, since `@Value` does not support relaxed binding, it is not a good candidate if you need to provide the value by using environment variables.
+Doing so will provide you with structured, type-safe object that you can inject into your own beans.
+
+If you do want to use `@Value`, we recommend that you refer to property names using their canonical form (kebab-case using only lowercase letters).
+This will allow Spring Boot to use the same logic as it does when relaxed binding `@ConfigurationProperties`.
+For example, `@Value("{demo.item-price}")` will pick up `demo.item-price` and `demo.itemPrice` forms from the `application.properties` file, as well as `DEMO_ITEMPRICE` from the system environment.
+If you used `@Value("{demo.itemPrice}")` instead, `demo.item-price` and `DEMO_ITEMPRICE` would not be considered.
 
 Finally, while you can write a `SpEL` expression in `@Value`, such expressions are not processed from <<boot-features-external-config-application-property-files,application property files>>.