Add a doc section about subtraction and hyphen

[srawlins]

Subtraction and hyphens tripped me up for a while today before I realized that margin: 2em-1em will not perform subtraction. I thought that since Division and / was so well documented, then it was the only exception to the operators. 😛 not so!

I'm not an authority on the statements I made in this change, but I used @nex3's explanation in #853 as guidelines.

[r4j4h]

+1 I could see this tripping someone else up too

[nex3]

I'm on board for documenting this, but I don't think this documentation gets at the heart of the matter. I'll try to come up with my own. It's also work mentioning that the behavior has changed a lot since #853; see #1030 and #1105.

[srawlins]

Thanks for the fix, @nex3 !