document what 'tahoe cp' does with trailing slashes

[davidsarah]

The --help text for tahoe cp says:

... Arguments should probably not have trailing slashes.
'tahoe cp' does not behave as much like /bin/cp as you
would wish, especially with respect to trailing slashes.

But this doesn't tell me how tahoe cp behaves with trailing slashes, and I also don't know how /bin/cp behaves with trailing slashes, so I don't know whether that is the behaviour I would want.

I looked at man cp on Linux to see how it documented cp's behaviour with trailing slashes, and it didn't say, apart from documenting this option:

   --strip-trailing-slashes
          remove any trailing slashes from each SOURCE argument