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