[doc] clarify behaviour of pg_dump's -t/--table option with non-tables

Hi

Recently I ran into a case where someone was wondering why it was not
possible to dump the contents of a view, even though the documentation [1]
seems to imply this is possible.

Currently it says:

Dump only tables with names matching pattern. For this purpose, "table"
includes views, materialized views, sequences, and foreign tables.

The attached patch attempts to clarify that only definitions of those objects
will be dumped, and also mentions that dumping foreign table data requires the
--include-foreign-data option.

I suggest backpatching any changes to Pg13 where the --include-foreign-data
option was added.

[1] https://www.postgresql.org/docs/current/app-pgdump.html

Regards

Ian Barwick

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