I guess you can find a lot of data on Functional Specification on the Net.
and Functional Specification dont have any thing to do with any single Module it is just a way of writing a Logic for the development or Enhance ment in such a way that an ABAP'er can read it and Understand the same, and also he should be able to develop the Program based on the Functional Specification.
Every Module Person has to wright an FS for the Customizations.
Hope it helps you.
A Functional Specification is a document you write when there is a business need (either during implementation, or as a change in the continous support & improvement cycles) for SAP to perform a certain functionality in a certain way to support a business process.
Once you receive a detailed requirement from the business, you write down a description of how this is going to be handled by SAP, and how should SAP be set up for this. This document should contain enough information for you to perform the necessary customization, or for a Technical analyst to develop the necessary enhancement.