Clarifying the docs about test.only

[jhbalestrin]

Summary

It was not clear that the test.only applies just to a single file, since the jest parallels the tests

Test plan

Just docs changes

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[codecov-io]

Codecov Report

Merging #5262 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5262   +/-   ##
=======================================
  Coverage   61.14%   61.14%           
=======================================
  Files         203      203           
  Lines        6843     6843           
  Branches        3        3           
=======================================
  Hits         4184     4184           
  Misses       2658     2658           
  Partials        1        1

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