Fix inference of return type of spectrogram/stft

The instability of the output type doesn't make a huge performance difference unless there are a large number of segments, but caused the return type of `spectrogram` to be inferred as `Union(Spectrogram{Float64,Frequencies},Spectrogram{Complex{Float64},Frequencies}))` which could be bad for user code that wants to process it further.

I removed the `psdonly` argument from the docs since I don't think there's much reason to use it directly.