Add details on k/v tombstone in blocking query section

Originally noted by @kaitlincarter-hc
https://www.consul.io/api/index.html#blocking-queries

"The short version is that when you delete keys we actually keep a “tombstone” around for 15 mins so that blocking queries on that key will still see monotonic (increasing) index values for long enough for them to complete their current/next request. It’s super subtle and doesn’t matter to most people though so I don’t know if it’s worth pulling into something central."