class CachedSpecialMethod: Fix formatting issue in docstring
cached_method: additional docstring

Added a docstring illustrating the different instances have distinct caches.
Trac #16621: cached_method: additional docstring

When reading the documentation, I was not fully sure that different the
caches of a {{{cached_method}}} of different instances are really
distinct. Therefore, I propose to include an example illustrating this
fact.

Along the way, a missing {{{::}}} in another docstring in the same file
has been added.

URL: http://trac.sagemath.org/16621
Reported by: cheuberg
Ticket author(s): Clemens Heuberger
Reviewer(s): Simon King