Improve pdumper doc; say unexec is deprecated

Say that pdumping cannot redump unless -batch is used.  Say that
the traditional unexec dumping method is by default not available,
and is deprecated.  Don't call dump files "portable", as dump files
are not any more portable than the Emacs executables themselves.
Just call them "dump files".  Similar, prefer "portable dumper"
(since the dumper code is portable) to "portable dumping" (since
the dump file is not).  Be more systematic about calling them
"dump files" instead of "dumped images" or whatnot.
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passed #2668