pgsql: Improve BRIN documentation somewhat

Improve BRIN documentation somewhat

This removes some info about support procedures being used, which was
obsoleted by commit db5f98ab4f, as well as add some more documentation
on how to create new opclasses using the Minmax infrastructure.
(Hopefully we can get something similar for Inclusion as well.)

In passing, fix some obsolete mentions of "mmtuples" in source code
comments.

Backpatch to 9.5, where BRIN was introduced.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8d9073692430a222939e7f2e436c80f0c271f116

Modified Files
--------------
doc/src/sgml/brin.sgml | 81 +++++++++++++++++++++++++++++++----
src/backend/access/brin/brin.c | 2 +-
src/backend/access/brin/brin_xlog.c | 6 +--
3 files changed, 77 insertions(+), 12 deletions(-)