pgsql: Explain subtlety in nbtree locking protocol.

Explain subtlety in nbtree locking protocol.

The Postgres approach to coupling locks during an ascent of the tree is
slightly different to the approach taken by Lehman and Yao. Add a new
paragraph to the "Differences to the Lehman & Yao algorithm" section of
the nbtree README that explains the similarities and differences.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/867d25ccb4c7f290d08c720622ecaae4afd1dc3f

Modified Files
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src/backend/access/nbtree/README | 27 ++++++++++++++++++++-------
src/backend/access/nbtree/nbtinsert.c | 3 +++
2 files changed, 23 insertions(+), 7 deletions(-)