Thanks for sharing your experiences too. I certainly will review this area to see if it can be made any clearer. One difficulty is that an “instrument change” in other software is something you have to think about and do consciously - in Dorico, that’s not really the case. You input the notes on the instrument staves that should have those notes, and if 2+ instruments are held by the same player, Dorico does the instrument change for you.
What there is in the manual already -
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On the page about instrument changes, a sentence saying: “Dorico Pro handles instrument changes automatically, including showing the appropriate instrument change labels, when you have input notes onto multiple instrument staves held by the same solo player, as long as the notes do not overlap.” Additionally at the bottom: “You can see staves for all instruments in galley view, and you can allow/disallow instrument changes in each layout independently.” There are then links at the bottom to switching to galley view and adding instruments to players.
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On the page about adding instruments to players (essentially the instructions for giving one player multiple instruments): “Before you have input any notes, only the first instrument held by solo players is shown in full scores in page view. All instrument staves are shown in galley view, so we recommend switching to galley view to input notes for any other instruments held by solo players.”
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As a prerequisite for inputting notes: “If you want to input notes into multiple instruments held by a single player or instruments not visible in the score in page view, you are in Galley View.”
Additionally, there is this tutorial video from 2016 -
