pgsql: Document BRIN a bit more thoroughly

Document BRIN a bit more thoroughly

The chapter "Interfacing Extensions To Indexes" and CREATE OPERATOR
CLASS reference page were missed when BRIN was added.  We document
all our other index access methods there, so make sure BRIN complies.

Author: Álvaro Herrera
Reported-By: Julien Rouhaud, Tom Lane
Reviewed-By: Emre Hasegeli
Discussion: https://www.postgresql.org/message-id/56CF604E.9000303%40dalibo.com
Backpatch: 9.5, where BRIN was introduced

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/c84c87ccc0e219a3f263aea9ec80068022ca1cc0

Modified Files
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doc/src/sgml/ref/create_opclass.sgml |   4 +-
doc/src/sgml/xindex.sgml             | 106 ++++++++++++++++++++++++++++++++++-
2 files changed, 106 insertions(+), 4 deletions(-)