Slack guidelines: revise self-promotion CTA language

[ddbeck]

Not everyone knows what an infomercial looks like. Here, we try to be more specific about what we're talking about.

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📚 Documentation preview 📚: https://writethedocs-www--2001.org.readthedocs.build/

[plaindocs]

I feel like first line and last line say pretty much the same thing, and would delete or consolidate, but don't feel strongly on the matter.

[janine-c]

Sorry about the delay on this one! Looks great - no one can accuse us of not giving enough detail this way 😄