Clarify PublicMetrics (uniqueness of metric names)

See gh-6404

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/PublicMetrics.java

@@ -22,9 +22,12 @@ import org.springframework.boot.actuate.metrics.Metric;
 
 /**
  * Interface to expose specific {@link Metric}s via a {@link MetricsEndpoint}.
+ * Implementations should take care that the metrics they provide have unique names in the
+ * application context, but they shouldn't have to care about global uniqueness in the JVM
+ * or across a distributed system.
  *
  * @author Dave Syer
- * @see SystemPublicMetrics
+ * @see SystemPublicMetrics SystemPublicMetrics for an example implementation
  */
 public interface PublicMetrics {