pgsql: Clarify error message and documentation related to typed tables.

Clarify error message and documentation related to typed tables.

We restrict typed tables (those declared as "OF composite_type")
to be based on stand-alone composite types, not composite types
that are the implicitly-created rowtypes of other tables.
But if you tried to do that, you got the very confusing error
message "type foo is not a composite type". Provide a more specific
message for that case. Also clarify related documentation in the
CREATE TABLE man page.

Erik Wienhold and David G. Johnston, per complaint from Hannu Krosing.

Discussion: https://postgr.es/m/CAMT0RQRysCb_Amy5CTENSc5GfsvXL1a4qX3mv_hx31_v74P==g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5d1d8b3c82c2a796bf7d89f2a5785ce9424acab1

Modified Files
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doc/src/sgml/ref/create_table.sgml | 16 ++++++++--------
src/backend/commands/tablecmds.c | 9 ++++++++-
src/test/regress/expected/typed_table.out | 7 ++++++-
src/test/regress/sql/typed_table.sql | 4 ++++
4 files changed, 26 insertions(+), 10 deletions(-)