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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5d97181335b7a6a4f4554b78a0ee31989891b817

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(-)