docstring_inherit decorator does not properly parse multi-line descriptions

[rabitt]

The docstring_inherit decorator isn't properly parsing multiline descriptions, and treats the second line as a new parameter. #419 removes the one multiline description from core.Dataset to temporarily avoid the problem, but opening this now in case it comes up again in the future.

[magdalenafuentes]

After discussing with @rabitt labeling this as docs since is not affecting anything at the moment. We'll have to keep it in mind for the future