Clarify which caches are automatically bound to the metrics registry

See gh-19412

Author: snicoll

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

@@ -1810,8 +1810,8 @@ The following cache libraries are supported:
 
 Metrics are tagged by the name of the cache and by the name of the `CacheManager` that is derived from the bean name.
 
-NOTE: Only caches that are available on startup are bound to the registry.
-For caches created on-the-fly or programmatically after the startup phase, an explicit registration is required.
+NOTE: Only caches that are configured on startup are bound to the registry.
+For caches not defined in the cache’s configuration, that is caches created on-the-fly or programmatically after the startup phase, an explicit registration is required.
 A `CacheMetricsRegistrar` bean is made available to make that process easier.