doc: Mention GIN indexes support parallel builds.

doc: Mention GIN indexes support parallel builds.

Commit 8492feb98f6 added support for parallel CREATE INDEX on GIN indexes.
However, previously two places in the documentation and two in the source
code comments still stated that only B-tree and BRIN indexes support
parallel builds.

This commit updates those references to correctly include GIN indexes.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Robert Treat <rob@xzilla.net>
Discussion: http://postgr.es/m/7d27d068-90e2-4022-9bd7-09b0fd3d4f47@oss.nttdata.com