Follow-up from "Add docs for ChatOps"

The following discussions from !5033 (merged) should be addressed:

@markpundsack started a discussion: (+4 comments)

Technically, I hope except: [chat] is implied for all other jobs? Is this documented in the YAML reference?
@markpundsack started a discussion: (+1 comment)

Is GITLAB_USER_ID guaranteed to be accurate? Can it be overridden by a project variable or variable in .gitlab-ci.yml for example?
@markpundsack started a discussion: (+3 comments)

I assume only output from script is sent back to Slack, and output from before_script and meta output is ignored. Either way, can this be clarified?

Edited Mar 21, 2018 by silv