Document strengths and weaknesses of Magnitude and Constant

[chiphogg]

Many things are easy for Magnitude based types that are virtually impossible for conventional numeric types --- for example, they can represent vast quantities without overflowing, and can even perform exact (reversible) symbolic math for arbitrary roots!

On the other hand, many seemingly basic tasks are virtually impossible for these types --- for example, addition, or inequality comparison.

This task is to make an Implementation Discussion page to explain these tradeoffs. We must also link to this page from the documentation for Magnitude and for Constant.