doc: consistent 'Returns:'

For consistency, changed all `Return:` to `Returns:` in the API docs.

PR-URL: https://github.com/nodejs/node/pull/9554


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>