Document System.Numerics APIs (#2912)

* Document System.Numerics APIs: op_Addition, op_Division, op_Multiply, op_Substraction, Substract

* suggestions by Jozkee and tannergooding

* Add individual remarks, apply suggestions by mairaw, Jozkee, tannergooding

* Method -> operation name

* suggestions by rpetrusha and Jozkee

Co-Authored-By: David Cantu <[email protected]>
Co-Authored-By: Ron Petrusha <[email protected]>

* suggestions by rpetrusha and Jozkee

* Tables with efficiency gain

* operator -> operation

* Remove cost tables

* remove unnecessary comment

Author: carlossanlop