I am confused about how to mark edits in the current Question or Answer. It seems to me that using EDIT: is a generally accepted standard, but I was not able to find anything "official" about it, as it is impossible to search for these terms.
Is it ok to add [UPDATE] to a question's title after a major revision is applicable not just to titles, but to post bodies. Stack Exchange uses revision histories instead of edit/update/revised marks within the text. Such marks should not be included, and should be edited out to make the text flow properly whenever seen.
The goal of Stack Exchange is to make questions and answers that are timeless. Adding "EDIT 2:" does not help with that goal at all.