tmpfiles: document that the "argument" field doesn't do quotes

tmpfiles: document that the "argument" field doesn't do quotes

This adjust the documentation to match the code, addressing #17740.

I actually think that not making the "argument" field accept quotes was
a mistake, but I also understand why this choice was made. Given that we
shipped this forever like this though I don't think it's worth changing
the behaviour now. Supporting quotes for this is not that important I
guess. Hence document the current behaviour.

Fixes: #17740