Adds documentation for respondImmediately
Adds documentation for respondImmediately

This completes the addition of the respondImmediately feature described
in cjohansen/Sinon.JS#285.

spinningarrow/sinon-docs,maxcal/sinon-docs,sinonjs/sinon-docs,sinonjs/sinon-docs,grassator/sinon-docs,spinningarrow/sinon-docs,sinonjs/sinon-docs,grassator/sinon-docs,chentsulin/sinon-docs,maxcal/sinon-docs,chentsulin/sinon-docs