ttl can return -1 or -2

When using ttl, it can return -1 if Key does not have an associated TTL or -2 if Key does not exists.
Improving Javadoc to mention it.

`redis_version:2.8.6`
```
> ttl test
(integer) -2

> set test "true"
OK

> ttl test
(integer) -1
```