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.
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.