Docstrings

[felixwang9817]

What this PR does / why we need it: This PR adds docstrings to public methods in public-facing classes (FeatureStore, FeatureView, Entity, etc.), and improves existing documentation.

Which issue(s) this PR fixes:

Fixes #1610

Does this PR introduce a user-facing change?:

Added docstrings

[codecov-commenter]

Codecov Report

Merging #1739 (681b126) into master (489a0f8) will decrease coverage by 0.04%.
The diff coverage is 79.36%.

@@            Coverage Diff             @@
##           master    #1739      +/-   ##
==========================================
- Coverage   79.08%   79.04%   -0.05%     
==========================================
  Files          80       80              
  Lines        6855     6880      +25     
==========================================
+ Hits         5421     5438      +17     
- Misses       1434     1442       +8     

Flag	Coverage Δ
integrationtests	78.96% <79.36%> (-0.05%)	⬇️
unittests	61.72% <77.77%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
sdk/python/feast/feature_service.py	71.21% <ø> (ø)
sdk/python/feast/feature_store.py	93.50% <0.00%> (ø)
sdk/python/feast/feature_table.py	44.05% <ø> (ø)
sdk/python/feast/feature_view.py	86.95% <62.50%> (-2.28%)	⬇️
sdk/python/feast/entity.py	73.98% <85.18%> (-2.60%)	⬇️
sdk/python/feast/data_source.py	57.00% <92.85%> (+0.81%)	⬆️
sdk/python/feast/feature.py	75.00% <100.00%> (ø)
sdk/python/feast/inference.py	89.79% <100.00%> (+0.21%)	⬆️
sdk/python/feast/infra/online_stores/dynamodb.py	93.67% <0.00%> (+2.53%)	⬆️

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[achals]

Looks good, mostly nits. Are doctests part of this PR or a different PR?

[felixwang9817]

Looks good, mostly nits. Are doctests part of this PR or a different PR?

Doctests are separate.

[achals]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: achals, felixwang9817

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