BRIN is missing in multicolumn indexes documentation

Hello,

the http://www.postgresql.org/docs/9.5/static/indexes-multicolumn.html
page doesn't mention BRIN support, but according to the
http://www.postgresql.org/docs/9.5/static/sql-createindex.html it is
supported in multicolumn setup.

The attached patch (git diff against master branch) fixes the omission
and adds "Oxford comma" before "and" to unify the writing style with
the style used in the "create index" documentation.

Regards,

Petr Jediny