ENH: Make MemorizedFunc __doc__ autodoc-compatible

This PR just strips the `\x08` control characters that `pydoc`  uses for displaying function names in bold, so that the modified docstring of a `MemorizedFunc` plays nicely with tools like `sphinx.ext.autodoc`.

Currently, using `autodoc` on `joblib.Memory('dir').cache(func(args))` results in documentation like this:

    Memoized version of ffuunncc(args)