Improving psql slash usage help message

psql slash usage show two options for listing foreign tables.

\dE[S+] [PATTERN] list foreign tables
\det[+] [PATTERN] list foreign tables

This seems a little odd especially when the output of both of these
commands is different.

postgres=# \dE+
List of relations
Schema | Name | Type | Owner
--------+------+---------------+--------
public | foo | foreign table | highgo
(1 row)

postgres=# \det
List of foreign tables
Schema | Table | Server
--------+-------+---------
public | foo | orc_srv
(1 row)

"\dE" displays the list with a "List of relations" heading whereas "\det"
displays "List of foreign tables". So, to differentiate the two, I suggest
to change the help message for "\dE" to:

\dE[S+] [PATTERN] list foreign relations

One could argue that both essentially mean the same thing, however,
considering "\dE+" also outputs size, it makes sense IMHO to make this
change (as PG documentation: relation is essentially a mathematical term
for table). Attached is the patch that makes this change.

Regards.

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