doc: Fix inconsistencies in GIN, BRIN and SP-GiST for optional opclass methods
The GIN and SP-GiST parts were out-of-sync since the changes of
14903f2,
and the BRIN part was wrong since its introduction in
15cb2bd.
Author: Guillaume Lelarge
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/CAECtzeXKvEPEr967h0PRYRi39uTmdEms=oUtc_PWGjZRNN1prw@mail.gmail.com
Backpatch-through: 13