chore: Updating documentation for entity's join key

[pyalex]

Signed-off-by: pyalex [email protected]

What this PR does / why we need it:

Following up on #2420

Which issue(s) this PR fixes:

Fixes #

[codecov-commenter]

Codecov Report

Merging #2451 (4b08731) into master (ea6a9b2) will increase coverage by 1.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2451      +/-   ##
==========================================
+ Coverage   83.66%   84.87%   +1.20%     
==========================================
  Files         125      127       +2     
  Lines       10838    11424     +586     
==========================================
+ Hits         9068     9696     +628     
+ Misses       1770     1728      -42     

Flag	Coverage Δ
integrationtests	75.60% <ø> (+2.31%)	⬆️
unittests	58.44% <ø> (+0.34%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdk/python/tests/unit/test_data_sources.py	87.50% <0.00%> (-12.50%)	⬇️
sdk/python/feast/repo_operations.py	47.42% <0.00%> (-7.96%)	⬇️
...gration/online_store/test_push_online_retrieval.py	100.00% <0.00%> (ø)
...ython/feast/embedded_go/online_features_service.py	95.08% <0.00%> (ø)
sdk/python/feast/feature_store.py	91.13% <0.00%> (+0.29%)	⬆️
...tegration/feature_repos/universal/feature_views.py	84.61% <0.00%> (+0.74%)	⬆️
...hon/feast/infra/offline_stores/snowflake_source.py	88.61% <0.00%> (+0.81%)	⬆️
...thon/feast/infra/offline_stores/bigquery_source.py	87.06% <0.00%> (+0.86%)	⬆️
...thon/feast/infra/offline_stores/redshift_source.py	87.06% <0.00%> (+0.86%)	⬆️
...ts/integration/feature_repos/repo_configuration.py	97.16% <0.00%> (+1.06%)	⬆️
... and 7 more

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[achals]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: achals, pyalex

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