Clarify feature-flag update pings for oncall/RMs

What does this MR do?

Adds a note that once a feature-flag rollout has started, it is not necessary to inform the @sre-oncall or @release-manager for every update.

This is in response to SRE feedback:

I’m getting a whole lot of feature flag requests today. IMO we should put some guidelines in place that if the feature flag is enabled for a single project or 5% only, then it is safe to enable without asking EOC.

Screenshots (strongly suggested)

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