Fix whitespace formatting in CLI help.

[Carreau]

For traitlets that have long descriptions, and in particular that uses multiline strings, this lead to inconsistant indentation – and mmissing newlines in rewrapped paragraphs.

• Each paragraph in wrap_paragraphs would be a list item, and thus need to be joined by 2 newlines (or we need to ensure that each list item in wrap_paragraph would end in a NL, but that woudl change API

• When help is ina multiline string, if the line start on the first line:

  """like in

  this kind of help"""

  but not like

  """
  in this kind

  of documentation help
  """

  Wrap parragraph would not properly dedent.

[minrk]

Thanks!