Adding docs for the members marked "To be added." in System.Math and System.MathF (#2519)

* Adding docs for the members marked "To be added." in System.Math and System.MathF

* Apply suggestions from code review

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

* Apply suggestions from code review

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

* Corrected escape character.

* Resolving remaining feedback

* Fixed badly formed XML.

Author: tannergooding