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