Technical documents to publish or not
It depends on whether the software is private or not. If it is public, you can either:


add your name to the AUTHORS file (you have one, right?).
ship the manual with the code.
slap a technical report number onto it, so that people will find it easier to cite.
write up a detailed description of what the software does, and publish it in a journal that publishes algorithms --- there are many of them.


If the code is a well-kept secret of your research group, not available even for sale, I am afraid there is very little you can do. People won't be interested in documentation of some code they cannot run themselves. You can still do 1--3, but don't expect people to read and cite it. 

Your only hope in that situation is that your co-authors are fair and acknowledge you appropriately whenever they use your code and documentation.