Docs: Missing --service option on install gitlab-runner command

Problem to solve

The option --service is missing from the documentation of command install for the gitlab-runner: https://docs.gitlab.com/11.10/runner/commands/README.html#gitlab-runner-install

Proposal

Add the option on the table where the options are listed.

Who can address the issue

Anyone on the gitlab-runner team

Other links/references

gitlab-runner#3688 (closed) - I needed to create a new service to avoid this kind of error and still have parallel builds