pgsql: Doc: Update the documentation for row movement behavior across p

Doc: Update the documentation for row movement behavior across partitions.

In commit f16241bef7c, we have changed the behavior for concurrent updates
that move row to a different partition, but forgot to update the docs.
Previously when an UPDATE command causes a row to move from one partition
to another, there is a chance that another concurrent UPDATE or DELETE
misses this row. However, now we raise a serialization failure error in
such a case.

Reported-by: David Rowley
Author: David Rowley and Amit Kapila
Backpatch-through: 11 where it was introduced
Discussion: https://postgr.es/m/CAKJS1f-iVhGD4-givQWpSROaYvO3c730W8yoRMTF9Gc3craY3w@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/793c736d69091d385a967b2740cc93cfb7a7b076

Modified Files
--------------
doc/src/sgml/ddl.sgml | 26 +++++++++++++-------------
1 file changed, 13 insertions(+), 13 deletions(-)