docs(assert): improve assertFalse() docs and test coverage

[tomas-zijdemans]

aligning tests with assert.ts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.08%. Comparing base (c3e47e8) to head (e028ce5).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6956   +/-   ##
=======================================
  Coverage   94.07%   94.08%           
=======================================
  Files         600      600           
  Lines       43528    43540   +12     
  Branches     6986     6991    +5     
=======================================
+ Hits        40950    40965   +15     
+ Misses       2524     2520    -4     
- Partials       54       55    +1     

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