Clarify extensions should live in specified namespaces #501

I noticed our newly introduced FFT stubs live in fourier_transform_functions.py, which might implicitly communicate those extensions should live in the xp.fourier_transform_functions namespace (xp being the top-level Array API namespace), where presumably we want adopters to use xp.fft like say NumPy has it.

So that can be renamed, but this brings up that there's no explicit clarification in the spec that the linalg and fft extensions should indeed live in those respective namespaces. I think a note could live in all extension pages (currently just Linear Algebra and Fourier transform Function) explaining that the respective functions should be present in these specific namespaces.