Documentation for IEntityMapper event handler?

[dalhaqq]

I was looking for how to change the value of a column automatically when an entity is created or another column is modified, then I found that IEntityMapper has an "on" method, but I can't find any documentation about it. I'm trying to read the code from the previous commit, but I still don't understand how it works
I don't know if it's appropriate to add an issue here or should I add it in the orm repository, but I think its good if there is an explanation about the method.
Thanks in advance.