Clean Singleton Docstrings

[Random832]

On Sat, Jul 16, 2016, at 03:27, Chris Angelico wrote:
> Will an "Exact" non-integer be stored as Decimal or
> Fraction? How do you know? They have vastly different semantics, and
> you should be able to choose.

Er, the point is for them to _not_ have different semantics. A decimal
storage format would simply be an optimization for a fraction whose
denominator is a power of 10 (or of 2 and 5)

The semantics of the current Decimal class are those of an inexact
number.