I'm not sure if there is documentation on where all of these widgets are. The Sitefinity API reference
in the SDK
does map out every class in the product, including all the web controls, so that might be helpful.
but your best bet is probably to use JustDecompile
a free tool from Telerik. you can open up the Telerik.Sitefinity dll and use the handy search feature to find the control you're seeking. from there you can see the namespace that contains it to reference on your control.
this is how I do it! hope this is helpful!