update Numeric/Float docstrings

update Numeric/Float docstrings

These docs were very out of date re: cdecimal.   Additionally,
as pointed out in #5252, the Numeric documentation is misleading;
SQLAlchemy's Numeric hierarchy resembles more of the Oracle
approach where precision and scale solely determine the kind of
datatype being worked with.   Float is essentially Numeric with
different defaults.

Fixes: #5252
Change-Id: I661109fabf04ba7831c7ddafba15321dd445ea5d