Here's what I did
I wrote a cover letter noting the reason for my request (ie: Advance Parole) and I listed the accompanying documentation that I included:
· Completed I-131 Application form
· A check payable to the “Immigration and Naturalization Service” in the amount of $110.
· A copy of the Receipt Notice of the pending I-485 application for Adjustment of Status.
· Two photographs, the A# and Name is written in pencil at the back of each.
ANd ended it off with something like 'I trust this meets the eligibility requirements for the renewal of the Advance Parole document etc...'
Note: the fees have changed since I sent mine in last week.
I know the section you are referring to, it wouldn't hurt to provide a reason. I did speak to the lawyer's office and the lady who worked on my original last year said she didn't include anything and that the I-485 pending is reason enough. And that is sort of what the instructions imply. I will wait and see.
good luck!
-harris