pgsql: Doc: be clearer that foreign-table partitions need user-added co

Doc: be clearer that foreign-table partitions need user-added constraints.

A very well-informed user might deduce this from what we said already,
but I'd bet against it.  Lay it out explicitly.

While here, rewrite the comment about tuple routing to be more
intelligible to an average SQL user.

Per bug #17395 from Alexander Lakhin.  Back-patch to v11.  (The text
in this area is different in v10 and I'm not sufficiently excited
about this point to adapt the patch.)

Discussion: https://postgr.es/m/17395-8c326292078d1a57@postgresql.org

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7d74ff82431ba8836f92ac33ed3f1ab276f51a92

Modified Files
--------------
doc/src/sgml/ddl.sgml                      |  9 ++++++---
doc/src/sgml/ref/create_foreign_table.sgml | 30 +++++++++++++++++++++++-------
2 files changed, 29 insertions(+), 10 deletions(-)