Fix ExpiryPagerComment

It incorrectly states that the task purges expired items from disk.

Change-Id: I4e0f18c81b6392d146c66cbda4838555fd09667f
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/163982
Tested-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>

Author: BenHuddleston

engines/ep/src/item_pager.h

@@ -132,8 +132,9 @@ class ItemPager : public GlobalTask {
 };
 
 /**
- * Dispatcher job responsible for purging expired items from
- * memory and disk.
+ * Dispatcher job responsible for expiring items currently stored in memory. The
+ * act of expiring (deleting) the item generally results in the deletion of the
+ * items value which saves memory and disk space.
  */
 class ExpiredItemPager : public GlobalTask,
                          public std::enable_shared_from_this<ExpiredItemPager> {