[dune-functions] Documenting BSplineBasis constructors

[Oliver Sander]

Dear C++ experts,
I just noticed that none of our basis implementations appears in the doxygen list-of-classes.
The reason is obvious: none of the is a class; they are all aliases to DefaultGlobalBasis.
That's still annoying though, because the list of classes is where many people start to look
for information.  It is particularly annoying for the BSplineBasis, which has non-trivial
constructors.  Any ideas on how to improve the situation?
Cheers,
Oliver