BUG: use the "join" string in Appender decorator

The docstring for Appender decorator says that "join" parameter is used to join the docstring and addendum.  This commit brings the code in line with the documentation.

Author: gandalf013

pandas/util/decorators.py

@@ -102,7 +102,7 @@ def __call__(self, func):
         func.__doc__ = func.__doc__ if func.__doc__ else ''
         self.addendum = self.addendum if self.addendum else ''
         docitems = [func.__doc__, self.addendum]
-        func.__doc__ = ''.join(docitems)
+        func.__doc__ = self.join.join(docitems)
         return func