Document data type of .on('data') callback argument

[jameshfisher]

What is the purpose of this pull request? (put an "X" next to item)

[x] Documentation update [ ] Bug fix [ ] New feature [ ] Other, please explain:

What changes did you make? (Give an overview)

Current documentation is vague. The user needs to know which type to expect.

Which issue (if any) does this pull request address?

Is there anything you'd like reviewers to focus on?

[jzombie]

Merged into https://github.com/zenOSmosis/webrtc-peer