Make it clear that `only:` and `except:` do not support a string value

What does this MR do?

Updates the documentation about only: and except: to make it clear that they accept an array, not a single value.

For example, only: merge_requests causes a GitLab CI configuration error:

This GitLab CI configuration is invalid: jobs:my-job:only has to be either an array of conditions or a hash.

This MR converts all mentions of things like only: merge_requests to only: [merge_requests].