The Dorico-approved way is to start a new flow. If you already have existing music after the time sig, you can use Write/Split Flow. You will likely then need to make quite a few adjustments in Layout Options/Page Setup/Flows to get the appearance the way you want. You will also probably want to set Layout Options/Indent First System to 0, then go to Engrave Mode and manually indent the first system of the piece. After that, change the bar numbering in your new flow to match the previous flow, then change the rehearsal letter index in the new flow. I’m not at my computer right now to check, but I seem to remember audio export is flow based too, so if you need to export the audio for a mockup, you’ll need to make sure you know how to use some sort of WAV editor too.
(Yes, Dan’s method is way easier. Even easier would be to just select the cautionary and hit Hide, but the developers seem strongly against that.)