document effect of USAGE on foreign server to dblink

I think that the permissions around the usage of foreign server names as
connection names in dblink should be documented, as in the attached.

Also, for dblink itself it says "It is recommended to use the foreign-data
wrapper dblink_fdw when defining the foreign server". Is there a reason
for this recommendation? postgres_fdw seems to work fine on the surface,
is there some hidden trap in using it?

Cheers,

Jeff