Fix/revert numpydoc docstrings

[brunnedu]

Summary

Updated docstrings for some statistics and time series generation functions to have docs look more uniform:

• reverted numpydoc docstring formatting (type hints in docstring)
• added references where links were previously in the description of the function

[hrzn]

Thanks

[codecov-commenter]

Codecov Report

Merging #744 (ff3f16f) into master (ddc84a2) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #744      +/-   ##
==========================================
- Coverage   90.80%   90.80%   -0.01%     
==========================================
  Files          67       67              
  Lines        6741     6740       -1     
==========================================
- Hits         6121     6120       -1     
  Misses        620      620              

Impacted Files	Coverage Δ
darts/utils/statistics.py	46.70% <ø> (ø)
darts/utils/timeseries_generation.py	95.49% <ø> (ø)
darts/utils/likelihood_models.py	96.04% <0.00%> (-0.01%)	⬇️

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