Correct docs about partitions and EXCLUDE constraints

Hi Hackers,

In pg17 we added support for cross-partition EXCLUDE constraints, as long as they included all 
partition key columns and compared them with equality (see 8c852ba9a4). I updated the docs for 
exclusion constraints, but I missed that the docs for CREATE TABLE still said that they were not 
supported. This commit fixes that. I don't see a similar message in the ALTER TABLE docs.

I take it we should apply this patch to both v18 and v17?

Yours,

-- 
Paul              ~{:-)
pj@illuminatedcomputing.com