DOC: new stats functions, docstrings need proof reading

[josef-pkt]

There are several errors and formatting problems (e.g. line breaks, lists)

e.g.

• https://www.statsmodels.org/dev/generated/statsmodels.stats.rates.tost_poisson_2indep.html
  Parameters, data for second sample copied "first", same for test_poisson_2indep
  Return has changed to HolderTuple

• test_poisson_2indep has etest_kwds in signature but missing in docstring, fixed but typos

• equivalence_scale_oneway has incomplete docstring, not Parameters, Return sections
  scale_transform "center" can also be a number, not included in docstring, but test_scale_oneway includes it correctly

• trim_mean docstring uses "distribution" instead of "observations" (I might have copied it from scipy, I used "fraction" instead of "proportiontocut")

• https://www.statsmodels.org/dev/generated/statsmodels.stats.robust_compare.TrimmedMean.html shows duplicate entry
  (incorrect rendering or autosummary)
  ignore, properties and attributes are twice by sphinx/autosummary
  I added parameter section to class docstring

• See Also sections are missing especially in closely related function, e.g. the various anova oneway functions

• https://www.statsmodels.org/stable/generated/statsmodels.stats.proportion.test_proportions_2indep.html says confidence interval in method description

[josef-pkt]

docstring for tost_poisson_2indep seems to have outdated Notes section method list
I guess TOST should work with all methods of tests with one-sided alternative

update looks fixed

[josef-pkt]

can be closed by upcoming doc fix PR