Update AttrDict docstring
Update AttrDict docstring

Co-authored-by: Adrian Sampson <a1b909ec1cc11cce40c28d3640eab600e582f833@radbox.org>
ibmibmibm/beets,sampsyo/beets,ibmibmibm/beets,beetbox/beets,shamangeorge/beets,jackwilsdon/beets,ibmibmibm/beets,sampsyo/beets,jackwilsdon/beets,beetbox/beets,shamangeorge/beets,sampsyo/beets,shamangeorge/beets,jackwilsdon/beets,shamangeorge/beets,jackwilsdon/beets,beetbox/beets,beetbox/beets,ibmibmibm/beets,sampsyo/beets