You have to complete your post first, then click the wrench under your post (not the general one on the right). You’ll get a pop up menu with “Add Staff Color”. It’s great, because you can retroactively make posts “official” if you need to.
Staff color looks like this, and requires a reply at the current bottom of the topic which may not be optimal.
I just noticed “Add Staff Notice” while making that screenshot, and discovered via experimentation that I can add a blue staff banner preambling any post. I’ll research, but I assume that’s visible to users (otherwise it’d just duplicate whisper functionality…) This is probably very useful for exactly what you say - like to label the first post “This topic is currently under moderation.” or similar.
Or perhaps this is a good place to put they “Hey behave.” type moderation messages, directly on the post. The “Don’t make me come in here, children…” type of soft warnings.
My view is that a moderator should not rewrite anyone’s posts, the only exception being to remove personal information (like a phone, credit card number, email, address) which they appear to have included inadvertently. I follow such an edit up with a PM explaining the privacy issues, and asking if they wish me to reverse the edit and let the information stand.
I also correct spelling in topic titles, and fix broken quotes and formatting, but that’s all.
If a post content is unacceptable then it needs flagging and hiding, and an exchange of PMs asking the user to rewrite or withdraw.
Which is cool - I discovered Discourse sends this private message automatically the first time it gets hidden by flags (unless you’ve altered this particular text in Admin settings).
Yes, that is also why we added staff notices / annotations which can be applied “above” a post to offer staff commentary visible to all.
I also edit entire topic titles in some cases, to draw more eyes. e.g. I might change “Please help” to “Help required with white labelling app”