tar-mode tiny doc additions

[Kevin Ryde <user42@zip.com.au>]

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I noticed tar-next-line and tar-previous-line lack docstrings,
something like below might be good, just so C-h k or an apropos can
say something nice.

Of course you're probably in trouble if you have to ask what C-n and
C-p do :-), but I don't use tar mode often, and when scrolled across
horizontally I was scratching my head for a moment why it kept jumping
back to the left.

Incidentally, I wonder if there should be a tar-mode-hook.  I don't
suppose there's much that could be usefully done from there, but I for
one would rebind C-n and C-p :-), as opposed to using evil-after-load.

	* tar-mode.el (tar-next-line, tar-previous-line): Add doc string, call
	the argument "arg" to match plain next-line and previous-line.


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