_decorators.Substitition fails on class docstring
```
from pandas.util._decorators import Substitution

@Substitution('Jason')
class Foo(object):
	"""Author is %s"""
	def __init__(self):
		pass
```

--> `AttributeError: attribute '__doc__' of 'type' objects is not writable`

This shouldn't be hard to fix, just parking it here for now.  The motivating use case is over in statsmodels where Model subclass docstrings can be really repetitive.
