I'm guessing the Shimano warning is mostly for the big chainring interference.
The Dealer Manual linked in the comment above is very good. All the steps are clearly explained, even for an amateur mechanic like me.
My Di2 trims the front derailleur automatically.,so there's no derailleur cage rubbing. (and the newer Di2 versions don't even allow the small-small cross chain.)
When cross chaining the small front and smallest two cogs, the chain hits the shifting assist pins on the side of the big chainring. Tick, tick, tick... It still rides okay, it's just an annoying noise.
Last edited by rm -rf; 05-22-23 at 06:15 AM.