Update doc of pairwise ttest: mixed analysis does not support two within-subject factors

[raphaelvallat]

Closes #454

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (65ae5e4) to head (aee7088).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #461   +/-   ##
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  Coverage   98.54%   98.54%           
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  Files          19       19           
  Lines        3360     3360           
  Branches      492      492           
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  Hits         3311     3311           
  Misses         26       26           
  Partials       23       23           

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