pgsql: 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.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5a2f154a2ecaf545000a3ff3cdbadc76ae1df30a

Modified Files
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src/backend/access/nbtree/README | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)