Update SI affix list in SFD_mathformatting.rst

@lamagnifica @catong @WatsonEmily Please review.

(DOC-1107: https://openedx.atlassian.net/browse/DOC-1107)
Agreed that it's confusing! @lamagnifica , I've shot you an email with more details since I'm not a markdown pro.

I think it might work to take a step back, and say "There are several shorthands for depicting very large numbers or very small numbers in the edX platform, including scientific notation, e notation, and using SI affixes".

Agreed that lots of examples will help, as well. 
**Scientific Notation**
4.351 * 10^5  = 435,100
4.351 * 10^-5 = 0.00004351
**e Notation (often found in calculators)**
     ‘e’ represents “times 10 raised to the following power of"
4.351e6 = 4,351,000   = 4.351 * 10^6
4.351e-2 = 0.04351     = 4.351 * 10-2
**Metric affixes**
4351m = 4.351
4.3M = 4,300,000


Finally, I think it's worth writing out the affixes explicity as 1k, 1M, 1G, etc.  because we have some variables doing double-duty, such as k meaning both kilo and boltzman's constant.  (i.e. 1k = 1000, 2k = 2000, k = 1.380e-23, 2*k = 2.761e-23)

Double-finally, I really like how this is getting clarified overall, and I've written a docs story to update the in-platform calculator tool tip once this is finalized! (https://github.com/edx/edx-platform/pull/1738) (https://openedx.atlassian.net/browse/DOC-1162)