>> we should be clearer about it in NEWS and documentation.
> 
> We do want to support re-dumping eventually, but we are clearly not
> there yet.

OK, I added something to that effect to the NEWS file and to internals.texi. 
While in the neighborhood, I also put in a statement that unexec is deprecated 
and rewrote instances of "portable dump file" as the dump files themselves are 
no more portable than an Emacs executable is. See attached patch.