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
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d0cd7b77265cf39e0e8984ba9d5bec0c155dccdb

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