`nbconvert`: Default help message should be that of --help

[dpsanders]

The help produced when nbconvert does not understand the input should be the output of --help, with the description in words of the solution.

(I was under the impression that this was the behaviour that had been agreed on.)

The help should discuss the nbconvert_build folder, if it remains.
I will add this discussion to my forthcoming PR on the description of the IPython Notebook.

[jakobgager]

I totally agree with you here! The first time I used the "new" nbconvert I had no clue where the new files got, till I found the nbconvert_build folder. This has to be added to the --help output!
👍

[ellisonbg]

Done.