Update comms-sign-off.mdx (#565)

docs/workflow/implementing/risk-mitigation/comms-sign-off/comms-sign-off.mdx

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-Under construction 🚧
+Ideally work with your comms or PR contact for your area to let them know the details of the experiment and why a risk has been identified.  If you don’t know where to start for Comms help for your area - #cccc is a channel where you can get help.  That channel also has a runbook document linked that has some more information on ways to engage with the Comms team.
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+It is very rare for any pro-active comms to come for an experiment - but we have had “reactive comms” ready around several user visible feature changes or messages.  Having reactive comms prepared and flagging the experiment for the support team makes responding to any incident quicker and less likely to escalate badly.
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