graft: clarify in help that `-r` is not just optional
Positional parameters are also treated as revisions, but the order of revisions
matters and it will often be wrong if the user understands it as `-r` taking
multiple revisions as `-r REV1 REV2`.
(Alternatively, `-r` could be turned into a no-op flag as the documentation
suggests. That would however be less "semantic markup" and I agree with the
implementation in 55e7f352b1d3 but not the documentation.)
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