Docs: concurrent builds of partitioned indexes are not supported

Document that CREATE INDEX CONCURRENTLY is not currently supported for
indexes on partitioned tables.

Discussion: https://postgr.es/m/CAKJS1f_CErd2z9L21Q8OGLD4TgH7yw1z9MAtHTSO13sXVG-yow@mail.gmail.com
Backpatch-through: 11