entering comments on budget line items

@Paul Kleeberg  No, there is no place to enter comments within the budget at this time.

But you're not the only one interested in this functionality. There is an existing Idea (enhancement) thread on this topic; take a moment to visit this thread to add your vote (in the yellow box under the first post, click the little arrow under the vote counter).

Additionally, this month's customer survey concerns future budget features, including this one, so you might want to take a minute to take that survey.