I think something like enforcing that would be something you need a pre-commit hook for in the repository itself; Merge just directly invokes
git commands for you behind the scenes.
That said, you could do something like apply a syntax that highlights commit messages in that format. That would visually distinguish things and at the same time also allow for showing as invalid any parts of a commit message that are not valid. I’m not sure offhand if such a syntax currently exists, though.
Regarding the second part of your question, there is official documentation but it doesn’t cover this. It works similar to Sublime except that it currently lacks the niceties to tell you what options you can apply without having to take extra steps yourself.