Deployment auto_merge defaults to true

See https://developer.github.com/v3/repos/deployments/#create-a-deployment
Remove `force` option from create_deployment docs

According to the docs, there is no such option:

https://developer.github.com/v3/repos/deployments/#create-a-deployment
auto_merge is a boolean, not string
document the task option
document the environment option
Merge pull request #528 from bgentry/deployment-doc-fixes

Deployment doc fixes