pgsql: Add documentation for opclass options - Mailing list pgsql-committers

Add documentation for opclass options

911e7020770 added opclass options and adjusted documentation for each
particular affected opclass.  However, documentation for extendability was
not adjusted.  This commit adjusts documentation for interfaces of index AMs
and opclasses.

Discussion: https://postgr.es/m/CAH2-WzmQnW6%2Bz5F9AW%2BSz%2BzEcEvXofTwh_A9J3%3D_WA-FBP0wYg%40mail.gmail.com
Author: Alexander Korotkov
Reported-by: Peter Geoghegan
Reviewed-by: Peter Geoghegan

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e6c6f427e356e3706ce2f0ae7e7e94e5501bbc13

Modified Files
--------------
doc/src/sgml/brin.sgml    |  30 +++++++++
doc/src/sgml/btree.sgml   |  33 ++++++++++
doc/src/sgml/gin.sgml     |  26 +++++++-
doc/src/sgml/gist.sgml    | 161 +++++++++++++++++++++++++++++++++++++++++++++-
doc/src/sgml/indexam.sgml |   2 +
doc/src/sgml/spgist.sgml  |  35 +++++++++-
doc/src/sgml/xindex.sgml  |  53 +++++++++++++++
7 files changed, 336 insertions(+), 4 deletions(-)