add text in documentation

src/main/asciidoc/reference/aerospike-object-mapping.adoc

@@ -66,7 +66,7 @@ The MappingAerospikeConverter can use metadata to drive the mapping of objects t
 In Spring Data Aerospike, documents come in two forms – non-versioned and versioned.
 Documents with an `@Version` annotation have a version field populated by the corresponding record’s generation count.
 Version can be passed to a constructor or not (in that case it stays equal to zero).
-* `@Expiration` - applied at the field level to mark a property to be used as expiration field.
+* `@Expiration` - applied at the field level to mark a property to be used as expiration field. The value must be effectively integer.
 Expiration can be specified in two flavors: as an offset in seconds from the current time or
 as an absolute Unix timestamp. Client system time must be synchronized with Aerospike server system time,
 otherwise expiration behaviour will be unpredictable.