[hibernate#2855] Add java doc to package-info.java

It reminds us which test case is using it, and it also handy
to use it with Spotless:
Spotless ignore pacakge-info.java files, but if we add javadoc
we can create a separate rule for applying the license header.

Author: DavideD

hibernate-reactive-core/src/test/java/org/hibernate/reactive/package-info.java

@@ -1,3 +1,6 @@
+/**
+ * Used in {@link org.hibernate.reactive.FilterTest}
+ */
 @FilterDef(name = "current", defaultCondition = "deleted = false")
 @FilterDef(name = "region", defaultCondition = "region = :region", parameters = @ParamDef(name = "region", type = org.hibernate.type.descriptor.java.StringJavaType.class))
 package org.hibernate.reactive;