According to cmt-firmware-rx51 as of PR 1.3 (the last firmware for the N900), we need the docs for version 10w08. Google doesn't show anything about them.

The cellmo-icpr82-headers package seems to be a 100% match to the cellular firmware as used in PR 1.3 though and there is no reason not to use those for reverse engineering, new code or whatever we want to (after all, the package was officially released by Nokia even if they didn't mean to and I seriously doubt the bit of Nokia owned by Microsoft and releasing Windows devices or the bit of Nokia not owned by Microsoft and releasing Android devices is going to care what we are doing with some random header files for an 8 year old obsolete Linux phone)