Document that sessions endpoint not available for reactive sessions

Closes gh-10882

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

@@ -107,7 +107,7 @@ The following technology-agnostic endpoints are available:
 
 |`sessions`
 |Allows retrieval and deletion of user sessions from a Spring Session-backed session
- store.
+ store. Not available when using Spring Session's support for reactive web applications.
 
 |`shutdown`
 |Lets the application be gracefully shutdown (not enabled by default).