Add docs for the new callsFake behavior

[lucasfcosta]

Purpose (TL;DR) - mandatory

This adds docs also containing an example to the callsFake method introduced in #1207.

Solution

I just added a new entry to the docs/release-source/release/stubs.md file for stub.callsFake with an example below its description.

I also made sure it was on top because I think this is one of the most used features for stubs we've got.

How to verify

1. Check out this branch (see github instructions below)
2. Go to the docs folder by running cd docs on Sinon's root folder
3. Ensure you have bundler by running gem install bundler
4. Install dependencies by running bundle install
5. Run a local server by running bundle exec jekyll serve
6. Generate a new version with npm version
7. Go to http://localhost:4000 and check the new docs for the next release

[coveralls]

Coverage decreased (-1.4%) to 93.963% when pulling bdd3f82 on lucasfcosta:callsFake-docs into e0cfccc on sinonjs:master.

[fatso83]

Thanks!