Document that tar archives are now properly terminated.

Document that tar archives are now properly terminated.

Commit 5a1007a5088cd6ddf892f7422ea8dbaef362372f changed the server
behavior, but I didn't notice that the existing behavior was
documented, and therefore did not update the documentation.
This commit does that.

I chose to mention that the behavior has changed rather than just
removing the reference to a deviation from a standard. It seemed
like that might be helpful to tool authors.

Discussion: http://postgr.es/m/CA+TgmoaYZbz0=Yk797aOJwkGJC-LK3iXn+wzzMx7KdwNpZhS5g@mail.gmail.com