[DOCS] Update docs for UnexpectedRowsExpectation

[NathanFarmer]

• Add Core language indicating that UnexpectedRowsExpectation doesn't need to be subclassed
• Add Core language indicating that {batch} keyword is optional and improve language describing what it does
• Add Cloud section for "Custom SQL Expectations"

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• PR title is prefixed with one of: [BUGFIX], [FEATURE], [DOCS], [MAINTENANCE], [CONTRIB]
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• Appropriate tests and docs have been updated

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.95%. Comparing base (f567c84) to head (b7009df).
Report is 1 commits behind head on develop.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #10391      +/-   ##
===========================================
- Coverage    79.98%   79.95%   -0.03%     
===========================================
  Files          459      459              
  Lines        39976    39976              
===========================================
- Hits         31974    31964      -10     
- Misses        8002     8012      +10     

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3.9 athena or clickhouse or openpyxl or pyarrow or project or sqlite or aws_creds	?
3.9 aws_deps	?
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3.9 mssql	?
3.9 mysql	?
3.9 postgresql	?
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