"\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.
help.c and the documentation need to be synchronized a bit more than this single issue.
Calling it "foreign relation" for \dE and "foreign table" for \det does convey that there is a difference - not sure it a huge improvement though. The "\d[Eimstv]" family of meta-commands should, in the help, probably be moved together to show the fact that they are basically "list relation names [of this type only]" while "\det" is "list foreign table info".
I think from a user perspective, grouping these wouldn't be helpful. For example, it may cause FDW related commands to be spread through out the help. Currently, those are nicely grouped together, which makes life relatively easy IMO.