Clarify CHANGELOG: TTL indexes must be on Time-ish field

As per the linked mongodb docs:

* the indexed field must be a date BSON type. If the field does not have a date type, the data will not expire.
* you cannot create this index on the _id field, or a field that already has an index.
* the TTL index may not be compound (may not have multiple fields).
* if the field holds an array, and there are multiple date-typed data in the index, the document will expire when the lowest (i.e. earliest) matches the expiration threshold.
* you cannot use a TTL index on a capped collection, because MongoDB cannot remove documents from a capped collection.