Doc: add an example of a self-referential foreign key to ddl.sgml.

While we've always allowed such cases, the documentation didn't
say you could do it.

Discussion: 
https://postgr.es/m/[email protected]

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bd9e46a1a4acafa170e9dbf35299518a216ffa41

Modified Files
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doc/src/sgml/ddl.sgml | 28 ++++++++++++++++++++++++++--
1 file changed, 26 insertions(+), 2 deletions(-)

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