[DOC] improve stats doc and warnings

[Remi-Gau]

closes #743
closes #745
closes #744

[codecov]

Codecov Report

Merging #746 (36165b1) into main (c661991) will increase coverage by 0.02%.
The diff coverage is 92.00%.

@@            Coverage Diff             @@
##             main     #746      +/-   ##
==========================================
+ Coverage   77.36%   77.38%   +0.02%     
==========================================
  Files         216      216              
  Lines        5562     5577      +15     
==========================================
+ Hits         4303     4316      +13     
- Misses       1259     1261       +2     

Flag	Coverage Δ
unittests	77.38% <92.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/defaults/checkOptions.m	96.33% <ø> (ø)
src/batches/stats/setBatchSubjectLevelGLMSpec.m	88.88% <84.61%> (-1.12%)	⬇️
src/IO/getData.m	93.93% <100.00%> (ø)
src/IO/overwriteDir.m	100.00% <100.00%> (ø)
src/bids_model/BidsModel.m	87.65% <100.00%> (ø)
src/messages/printProcessingSubject.m	100.00% <100.00%> (ø)
src/workflows/stats/bidsFFX.m	76.38% <100.00%> (+0.67%)	⬆️
src/workflows/stats/bidsRFX.m	84.84% <100.00%> (+0.47%)	⬆️
src/workflows/stats/bidsResults.m	45.87% <100.00%> (+0.50%)	⬆️

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