add a documentation note for project displayName configuration

[ronami]

Summary

This PR addresses issue: #4379. Please let me know if something should be changed.

Test plan

Simply docs, no tests 🙂

[codecov-io]

Codecov Report

Merging #5600 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5600      +/-   ##
==========================================
+ Coverage   60.62%   60.63%   +<.01%     
==========================================
  Files         213      213              
  Lines        7312     7313       +1     
  Branches        3        3              
==========================================
+ Hits         4433     4434       +1     
  Misses       2878     2878              
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-config/src/normalize.js	93.4% <0%> (ø)	⬆️
packages/jest-cli/src/search_source.js	45.83% <0%> (+0.76%)	⬆️

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[rickhanlonii]

Looks great, thanks!

[github-actions]

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