doc: Mention BRIN indexes support parallel builds
Two places in the documentation suggest B-tree is the only index access
method allowing parallel builds. Commit
b4375717 added parallel builds
for BRIN too, but failed to update the docs. So fix that, and backpatch
to 17, where parallel BRIN builds were introduced.
Author: Egor Rogov
Backpatch-through: 17
Discussion: https://postgr.es/m/
114e2d5d-125e-07d8-94aa-
5ad175fb7443@postgrespro.ru