fix: always use half of available parallelism

[mrazauskas]

Not sure if that was intentional. While calculating concurrency, current logic is takes half of availableParallelism(), but os.cpus().length is not divided by 2. So I though to fix this.

Also documentation needs an adjustment.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.70%. Comparing base (cf22b0a) to head (84573ba).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #611      +/-   ##
==========================================
- Coverage   98.90%   97.70%   -1.20%     
==========================================
  Files          26       33       +7     
  Lines        1646     1788     +142     
  Branches        9       15       +6     
==========================================
+ Hits         1628     1747     +119     
- Misses         12       26      +14     
- Partials        6       15       +9     

Flag	Coverage Δ
linux	?
osx	97.70% <100.00%> (-0.90%)	⬇️
windows	?

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[wellwelwel]

There is no need for immediate release or so.

Thanks @mrazauskas, especially for updating documentation. I'm fixing the #608, I'll release both together 🙋🏻‍♂️