pgsql: doc: move PARTITION OF stanza to just below PARTITION BY

doc: move PARTITION OF stanza to just below PARTITION BY

It's more logical this way, since the new ordering matches the way the
tables are created; but in any case, the previous location of PARTITION OF
did not appear carefully chosen anyway (since it didn't match the
location in which it appears in the synopsys either, which is what we
normally do.)

In the PARTITION BY stanza, add a link to the partitioning section in
the DDL chapter, too.

Suggested-by: David G. Johnston
Discussion: https://postgr.es/m/CAKFQuwY4Ld7ecxL_KAmaxwt0FUu5VcPPN2L4dh+3BeYbrdBa5g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c6736ff76046521f56c50deb31da218bc1b29533

Modified Files
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doc/src/sgml/ref/create_table.sgml | 273 +++++++++++++++++++------------------
1 file changed, 139 insertions(+), 134 deletions(-)