Improve documentation for `spy(originalFunction)`
Improve documentation for `spy(originalFunction)`

Improve the wording. The previous version was plain wrong.

fixes #2162
mroderick/Sinon.JS,mroderick/Sinon.JS,mroderick/Sinon.JS,cjohansen/Sinon.JS,cjohansen/Sinon.JS,cjohansen/Sinon.JS