pgsql: Doc: Describe CREATE INDEX deduplication strategy.

Doc: Describe CREATE INDEX deduplication strategy.

The B-Tree index deduplication strategy used during CREATE INDEX and
REINDEX differs from the lazy strategy used by retail inserts.  Make
that clear by adding a new paragraph to the B-Tree implementation
section of the documentation.

In passing, do some copy-editing of nearby deduplication documentation.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/449e14a5618432f01066c33055229b96666bd925

Modified Files
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doc/src/sgml/btree.sgml | 54 +++++++++++++++++++++++++++++++++----------------
1 file changed, 37 insertions(+), 17 deletions(-)