Given that the vendors provide their own complete documentation for their implementations, one might ask why embark on this adventure at all? The answer is both principled and practical.
First, this is our implementation, and to be complete it needs independent documentation. Unlike the vendors' offerings, ours offers some features from each version, while lacking others. When we have been forced to choose between documented behaviors, the FreeTDS user is presumably best served by the FreeTDS reference manual.
Second, while the vendors today freely offer their documentation on the world wide web, that might not – in fact, surely will not – always be the case. Should the vendor decide to drop its offering for whatever reason, it will likely pull its documentation, too, sooner or later. Free sofware, in short, can lean on the capitalist model for support, but it can never rely on it.
This manual is incomplete, and is being completed in fits and starts. The focus to date has been on db-lib and the underlying network library, libtds. It's the kind of enormous Quixoitic undertaking that you'd reasonably doubt will ever be completed. If you feel inclined to jump in, please don't hesitate.
The organization of the manual could be better. We're still learning how to use Doxygen to best effect.
Probably the most useful link at the moment is "Modules", above. That gives a categorized, alphabetical listing of the documented functions.