pgsql: nbtree README: move VACUUM linear scan section.

nbtree README: move VACUUM linear scan section.

Discuss VACUUM's linear scan after discussion of tuple deletion by
VACUUM, but before discussion of page deletion by VACUUM.  This
progression is a lot more natural.

Also tweak the wording a little.  It seems unnecessary to talk about how
it worked prior to PostgreSQL 8.2.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/128dd901a5c87e11c6a8cbe227a806cdc3afd10d

Modified Files
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src/backend/access/nbtree/README | 55 ++++++++++++++++++++--------------------
1 file changed, 28 insertions(+), 27 deletions(-)