Name the arithmetic operators, including modulo, in the Reference

I write the name of the “modulo” operator because I wanted to know how to do it in Sass, but couldn’t find out by searching the Sass Reference. Searching for `%` didn’t help, since that character is used all over in percentages.

I added the names of the other operators because the list would look weird if only one of the operators were named. And I suppose somebody might want to search for “division”, if they noticed that `/` is already used in CSS and wondered if Sass used a different character for division.
Merge pull request #898 from roryokane/patch-2

Name the arithmetic operators, including modulo, in the Reference