pgsql: Add documentation for opclass options

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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master

Details
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https://git.postgresql.org/pg/commitdiff/15cb2bd27009f73a84a35c2ba60fdd105b4bf263

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(-)