pgsql: Fix omission in partitioning limitation documentation

Fix omission in partitioning limitation documentation

UNIQUE and PRIMARY KEY constraints can be created on ONLY the
partitioned table.  We already had an example demonstrating that,
but forgot to mention it in the documentation of the limits of
partitioning.

Author: Laurenz Albe
Reviewed-By: shihao zhong, Shubham Khanna, Ashutosh Bapat
Backpatch-through: 12
Discussion: https://postgr.es/m/167299368731.659.16130012959616771853@wrigleys.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ba2d2784f3f83277de6f9c17505229fa3f2f6d5c

Modified Files
--------------
doc/src/sgml/ddl.sgml | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)