Clarify why _bt_killitems sorts its items array.

Clarify why _bt_killitems sorts its items array.

Make it clear why _bt_killitems sorts the scan's so->killedItems[]
array.  Also add an assertion to the _bt_killitems loop (that iterates
through this array) to verify it accesses tuples in leaf page order.

Follow-up to commit bfb335df58.

Author: Peter Geoghegan <[email protected]>
Suggested-by: Victor Yegorov <[email protected]>
Discussion: https://postgr.es/m/CAGnEboirgArezZDNeFrR8FOGvKF-Xok333s2iVwWi65gZf8MEA@mail.gmail.com