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

Author: alvherre

doc/src/sgml/ddl.sgml

@@ -3856,27 +3856,6 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
       </para>
      </listitem>
 
-     <listitem>
-      <para>
-       When an <command>UPDATE</command> causes a row to move from one
-       partition to another, there is a chance that another concurrent
-       <command>UPDATE</command> or <command>DELETE</command> will get a
-       serialization failure error.  Suppose session 1 is performing an
-       <command>UPDATE</command> on a partition key, and meanwhile a concurrent
-       session 2 for which this row is visible performs an
-       <command>UPDATE</command> or <command>DELETE</command> operation on this
-       row.  In such case, session 2's <command>UPDATE</command> or
-       <command>DELETE</command>, will detect the row movement and raise a
-       serialization failure error (which always returns with an SQLSTATE code
-       '40001').  Applications may wish to retry the transaction if this
-       occurs.  In the usual case where the table is not partitioned, or where
-       there is no row movement, session 2 would have identified the newly
-       updated row and carried out the
-       <command>UPDATE</command>/<command>DELETE</command> on this new row
-       version.
-      </para>
-     </listitem>
-
      <listitem>
       <para>
        <literal>BEFORE ROW</literal> triggers, if necessary, must be defined

doc/src/sgml/ref/create_foreign_table.sgml

@@ -160,6 +160,18 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>PARTITION OF <replaceable>parent_table</replaceable> FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable></literal></term>
+    <listitem>
+     <para>
+      This form can be used to create the foreign table as partition of
+      the given parent table with specified partition bound values.
+      See the similar form of
+      <xref linkend="sql-createtable"/> for more details.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>CONSTRAINT <replaceable class="parameter">constraint_name</replaceable></literal></term>
     <listitem>
@@ -308,6 +320,12 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
     responsibility to ensure that the constraint definition matches
     reality.
    </para>
+
+   <para>
+    While rows can be moved from local partitions to a foreign-table partition
+    (provided the foreign data wrapper supports tuple routing), they cannot be
+    moved from a foreign-table partition to another partition.
+   </para>
  </refsect1>
 
  <refsect1 id="sql-createforeigntable-examples">

doc/src/sgml/ref/update.sgml

@@ -287,13 +287,30 @@ UPDATE <replaceable class="parameter">count</replaceable>
    row satisfies its partition constraint, then the row is moved to that
    partition. If there is no such partition, an error will occur.  Behind the
    scenes, the row movement is actually a <command>DELETE</command> and
-   <command>INSERT</command> operation. However, there is a possibility that a
-   concurrent <command>UPDATE</command> or <command>DELETE</command> on the
-   same row may miss this row. For details see the section
-   <xref linkend="ddl-partitioning-declarative-limitations"/>.
-   Currently, rows cannot be moved from a partition that is a
-   foreign table to some other partition, but they can be moved into a foreign
-   table if the foreign data wrapper supports it.
+   <command>INSERT</command> operation.
+  </para>
+
+  <para>
+   There is a possibility that a concurrent <command>UPDATE</command> or
+   <command>DELETE</command> on the row being moved will get a serialization
+   failure error.  Suppose session 1 is performing an <command>UPDATE</command>
+   on a partition key, and meanwhile a concurrent session 2 for which this
+   row is visible performs an <command>UPDATE</command> or
+   <command>DELETE</command> operation on this row.  In such case,
+   session 2's <command>UPDATE</command> or <command>DELETE</command> will
+   detect the row movement and raise a serialization failure error (which
+   always returns with an SQLSTATE code '40001').  Applications may wish to
+   retry the transaction if this occurs.  In the usual case where the table
+   is not partitioned, or where there is no row movement, session 2 would
+   have identified the newly updated row and carried out the
+   <command>UPDATE</command>/<command>DELETE</command> on this new row
+    version.
+  </para>
+
+  <para>
+   Note that while rows can be moved from local partitions to a foreign-table
+   partition (provided the foreign data wrapper supports tuple routing), they
+   cannot be moved from a foreign-table partition to another partition.
   </para>
  </refsect1>