There really isn't any difference between the first time or the second time you choose to offer a bounty.
The Bounty interface already includes a link to the Bounty FAQ.
The first line is this
What if I don't get a good answer?
In order to get good answers, you have to put some effort into your question. Edit your question to provide status and progress updates. Document your own continued efforts to answer your question. This will naturally bump your question and get more people interested in it.
If, despite your best efforts, you feel questions aren't getting good answers, you can help by [offering a bounty].
One of the tenets of a smooth user interface is that as often as possible you should present only the information that is absolutely required.
For instance, the information that appears every time you ask a question is required because questions are the backbone of the site, if people ask terrible questions, the site suffers.
If, however, users offer ill-conceived bounties, it only hurts them. If they feel this is unfair, they should absolutely come to meta, and the people here will helpfully guide them towards how to improve their question. Although, I can't promise that we won't be snarky about it. (happens from time to time)
Extra help text would clutter the relatively sleek and simple Bounty interface and is not necessary.