pgsql: Document BRIN's inclusion opclass framework

Document BRIN's inclusion opclass framework

Backpatch to 9.5 -- this should have been part of b0b7be61337, but we
didn't have 38b03caebc5de either at the time.

Author: Emre Hasegeli
Revised by: Ian Barwick
Discussion:
 http://www.postgresql.org/message-id/CAE2gYzyB39Q9up_-TO6FKhH44pcAM1x6n_Cuj15qKoLoFihUVg@mail.gmail.com
 http://www.postgresql.org/message-id/562DA711.3020305@2ndquadrant.com

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3e9e03353966efa5cae5f927a77ba64a93f1ee8c

Modified Files
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doc/src/sgml/brin.sgml |  196 ++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 191 insertions(+), 5 deletions(-)