Update Configuration.md

[rafalp]

Summary

This PR adds note to globals setting that its contents need to be json serializable, and thus its not supporting functions for its values. This is something that originally has bitten me once I've moved to parallelized tests execution and it feels to me that this should be documented somewhere.

Test plan

This is documentation's change.

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[codecov-io]

Codecov Report

Merging #5287 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5287   +/-   ##
=======================================
  Coverage   60.98%   60.98%           
=======================================
  Files         203      203           
  Lines        6816     6816           
  Branches        4        3    -1     
=======================================
  Hits         4157     4157           
  Misses       2658     2658           
  Partials        1        1

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[cpojer]

Thank you!

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