[FEA] auto-merge bot should post instructions for resolving merge conflict when merge fails

[jlowe]

The auto-merge bot posts a PR to automatically merge, but when the merge fails it simply says, "Manual operation is required." It would be better to have that comment include the steps needed to post a PR that will resolve the merge conflict and also mention that the resulting PR needs to be merge committed rather than squashed. Bonus points if the message is parameterized to include the appropriate branch names (e.g.: a merge PR trying to upmerge branch-21.12 with branch-21.10 that fails with a merge conflict mentions those two branches in the instructions on how to resolve the conflict).

[pxLi]

I will add a how-to in the bot comment