For smc.py, add to the attributes section under SMC_KERNEL class

[rowan-schaefer]

I would add an attributes section here at the bottom of the docstring. They look similar to the parameters section: https://numpydoc.readthedocs.io/en/latest/format.html#documenting-classes. Start listing the self.xxx listed in the description, we'll later find someone to fill them out (e.g. _initialize_kernel section in the description mentions self.tempered_posterior and populates the self.var_info.

Originally posted by @OriolAbril in #6114 (comment)