update doc and loo recommendations

[avehtari]

Small updates to doc. I decided to keep the k_threshold argument default unchanged (0.7), as here it specifically controls for which observations to run moment_matching or reloo, but loo recommendations has a new recommendation if the sample size specific threshold is <0.7 and there are Pareto k's exceeding that threshold. Unfortunately, ps_khat_threshold() is not yet exported in loo or posterior so had to copy that.

EDIT:fixes #1606

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.94%. Comparing base (26a80a4) to head (b7fdca2).

Files	Patch %	Lines
R/loo.R	88.88%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1624      +/-   ##
==========================================
- Coverage   81.94%   81.94%   -0.01%     
==========================================
  Files          70       70              
  Lines       19951    19956       +5     
==========================================
+ Hits        16348    16352       +4     
- Misses       3603     3604       +1     

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[paul-buerkner]

Thanks! Merging now.