In the interest of giving credit where credit is due, my avatar is a scan of a hand-drawn caricature by cartoonist Jim Naylor which was done at my company's 20th anniversary dinner.
Notice on page 4 of the rules under Place a minion, it states You take one of your minions and you place it in an area on the board.....
Trolls and Demons are also explained on the last page (summary of random events) wherein it explains they are placed via the random event, but can be moved or removed from the board in the same way as minions.
And added to that the fact that it also says These rules apply whenever you have to place a minion shows that this is meant to apply to card text that says place a minion as well as cards with the place a minion symbol on them.
Although I also assume that where the card text specifies some additional restrictions then that overrides the relevant part of the place a minion rules - for example when a card says to place a minion in the Unreal Estate I assume that you do so (placing one of your own minions) even if you have no minions in an adjacent area.