[doc] Add note about test dependencies in --repeat option docs

[vkarak]

Fixes #2558.

[codecov-commenter]

Codecov Report

Merging #2564 (255f9d4) into master (64bfe9f) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2564      +/-   ##
==========================================
- Coverage   86.04%   86.03%   -0.01%     
==========================================
  Files          59       59              
  Lines       11030    11030              
==========================================
- Hits         9491     9490       -1     
- Misses       1539     1540       +1     

Impacted Files	Coverage Δ
reframe/frontend/executors/policies.py	92.30% <0.00%> (-0.29%)	⬇️

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