Update numbers module documentation

[cossor]

Documentation

The example in this implementation note became obsolete in 2010 (dc787d2). Numeric type implementers should study the implementations of classes like Decimal and Fraction instead of relying on this web page, so I think removing this note would be better than updating it.

[picnixz]

The example is about Fraction itself. Is it now outdated? If so, we should update it, if not, we should not.