For modules that are part of Linux (i.e. distributed with the base kernel), you can sometimes find documentation in the Documentation subdirectory of the Linux source code.
Many LKMs can be alternatively bound into the base kernel. If you do that, you will pass parameters to them via the kernel "command line," which in its most basic form means via a prompt at boot time. The BootPrompt HOWTO by Paul Gortmaker <Paul.Gortmaker@anu.edu.au> will help you with that. It is available from the Linux Documentation Project.
Don't forget that the source code of Linux and any LKM is always the documentation of last resort, and the most trustworthy.