Improve documentation for distributions

[Ahanmr]

Shows DeprecationWarning when sd is used instead of sigma

bug: #3688

[codecov]

Codecov Report

Merging #3837 into master will decrease coverage by 0.05%.
The diff coverage is 35.00%.

@@            Coverage Diff             @@
##           master    #3837      +/-   ##
==========================================
- Coverage   90.79%   90.73%   -0.06%     
==========================================
  Files         135      135              
  Lines       21113    21130      +17     
==========================================
+ Hits        19170    19173       +3     
- Misses       1943     1957      +14     

Impacted Files	Coverage Δ
pymc3/distributions/timeseries.py	70.89% <0.00%> (-0.76%)	⬇️
pymc3/distributions/continuous.py	80.10% <23.07%> (-0.76%)	⬇️
pymc3/distributions/mixture.py	87.27% <50.00%> (-0.35%)	⬇️
pymc3/distributions/multivariate.py	78.93% <100.00%> (ø)
pymc3/step_methods/hmc/base_hmc.py	95.32% <0.00%> (-0.94%)	⬇️

[twiecki]

Yes, I think it's time. @fonnesbeck @junpenglao @ColCarroll et al what do you think?

[junpenglao]

I think it is time as well.

[twiecki]

Ah, forgot that we need a line in the release notes, can you add that @Ahanmr in a separate PR?

[Ahanmr]

@twiecki Yeah, I'll that in the release notes as a point in Maintenance, for version 3.9?

[twiecki]

@Ahanmr yep.