[Backport][2.x] Add a link to the flakey test dev guide.

[dblock]

Signed-off-by: Daniel (dB.) Doubrovkine [email protected]

Description

Backport of #4872.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/5427/
• CommitID: a164d2f

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/5431/
• CommitID: 5c1c92a

[codecov-commenter]

Codecov Report

Merging #4894 (5c1c92a) into 2.x (5bf2ec4) will increase coverage by 0.18%.
The diff coverage is 71.76%.

@@             Coverage Diff              @@
##                2.x    #4894      +/-   ##
============================================
+ Coverage     70.56%   70.75%   +0.18%     
- Complexity    57592    57695     +103     
============================================
  Files          4620     4620              
  Lines        276152   276198      +46     
  Branches      40409    40418       +9     
============================================
+ Hits         194879   195420     +541     
+ Misses        64965    64481     -484     
+ Partials      16308    16297      -11     

Impacted Files	Coverage Δ
...in/java/org/opensearch/index/shard/IndexShard.java	69.70% <56.66%> (-0.50%)	⬇️
...ch/indices/recovery/PeerRecoveryTargetService.java	55.48% <66.66%> (+2.95%)	⬆️
...search/indices/recovery/RecoverySourceHandler.java	78.39% <83.78%> (+3.10%)	⬆️
...org/opensearch/index/shard/IndexShardTestCase.java	93.70% <100.00%> (+0.03%)	⬆️
...a/org/opensearch/client/cluster/ProxyModeInfo.java	0.00% <0.00%> (-57.90%)	⬇️
...cluster/coordination/PendingClusterStateStats.java	20.00% <0.00%> (-48.00%)	⬇️
...opensearch/persistent/PersistentTasksExecutor.java	22.22% <0.00%> (-44.45%)	⬇️
...ations/bucket/terms/heuristic/PercentageScore.java	17.39% <0.00%> (-34.79%)	⬇️
...earch/index/fielddata/SortingNumericDocValues.java	57.14% <0.00%> (-28.58%)	⬇️
.../opensearch/transport/ProxyConnectionStrategy.java	62.59% <0.00%> (-23.67%)	⬇️
... and 478 more

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