Improve nbtree README's LP_DEAD section.

Improve nbtree README's LP_DEAD section.

The description of how LP_DEAD bit setting by index scans works
following commit 2ed5b87f was rather unclear.  Clean that up a bit.

Also refer to LP_DEAD bit setting within _bt_check_unique() at the start
of the same section.  This mechanism may actually be more important than
the generic kill_prior_tuple mechanism that the section focuses on, so
it at least deserves to be mentioned in passing.