Document create_distributed_table_concurrently()

[Denton-L]

Currently, the documentation for the Citus API only includes create_distributed_table() and not create_distributed_table_concurrently0. The function in question is only documented in the README1 and in a blog post2.

We should document the function in the Citus API docs for better discoverability.

[onurctirtir]

Thank you for creating the issue.

I feel like we might have missed documenting more UDFs / GUCs / Citus views etc. like this, so we might want to go over the releases >= 12.1 to make sure.