Document difference between "Update Issue" and "Bump!" (BB-4307)

When commenting on an existing ticket, there are two buttons: "Update issue" and "Bump!".

However, it's not very clear what the difference is between those two. I found [[http://confluence.atlassian.com/display/BITBUCKET/Using+your+bitbucket+Issue+Tracker|this help page]] but it wasn't very helpful.

I'm guessing that "Bump!" is like "Update issue" just without the need to actually enter a comment, right? If so, perhaps the "Update issue" button should read "Bump!" as long as the user hasn't changed any details nor typed a comment.

Otherwise the difference should be documented somewhere (maybe in a tooltip).