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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master

Details
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http://git.postgresql.org/pg/commitdiff/a3a8309d450f7c4d1b743e84ba54ef5f7877d7be

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(-)