pgsql: Fix documentation on partitioning vs. foreign tables

Fix documentation on partitioning vs. foreign tables

1. The PARTITION OF clause of CREATE FOREIGN TABLE was not explained in
the CREATE FOREIGN TABLE reference page. Add it.
(Postgres 10 onwards)

2. The limitation that tuple routing cannot target partitions that are
foreign tables was not documented clearly enough. Improve wording.
(Postgres 10 onwards)

3. The UPDATE tuple re-routing concurrency behavior was explained in
the DDL chapter, which doesn't seem the right place. Move it to the
UPDATE reference page instead. (Postgres 11 onwards).

Authors: Amit Langote, David Rowley.
Reviewed-by: Etsuro Fujita.
Reported-by: Derek Hans
Discussion: https://postgr.es/m/CAGrP7a3Xc1Qy_B2WJcgAD8uQTS_NDcJn06O5mtS_Ne1nYhBsyw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fc84c05acd151cb1141c47d4af0c5ca803a4c3b4

Modified Files
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doc/src/sgml/ddl.sgml | 21 --------------------
doc/src/sgml/ref/create_foreign_table.sgml | 18 +++++++++++++++++
doc/src/sgml/ref/update.sgml | 31 +++++++++++++++++++++++-------
3 files changed, 42 insertions(+), 28 deletions(-)