Where to find GitLab Mattermost settings summary?

Hello,

I've installed GitLab Omnibus CE 8.13.3 and would like to understand the different settings in gitlab.rb for mattermost.

I am trying to change the notifications on GitLab events that gets published to our Mattermost channel, right now the notification is coming from "root" even though we have a different username defined in the outgoing web hook in GitLab.

Our gitlab.rb has

# mattermost['service_enable_post_username_override'] = false

Where can I find what service_enable_post_username_override means? I'm wondering if this setting is related. Plus I want to understand what all these different settings are.