Issue #162 clarify the use of missing labels

[YipengHu]

Description

Edited the doc to clarify the use of all-zero masks for missing data, instead of the alternatives with "automated handling missing data" which may be considered error-prone without clear benefit.

Fixes #162

Type of change

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• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Documentation Update (fix or improvement on the documentation)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Other (if none of the other choices apply)

Checklist

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• [ ] I have installed pre-commit using pre-commit install and formatted all changed files. If you are not certain, run pre-commit run --all-files.
• [ ] My commits' message styles matches our requested structure, e.g. Issue #<issue number>: detailed message.
• [ ] I have updated the change log file regarding my changes.
• [ ] I have added tests that prove my fix is effective or that my feature works
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[codecov[bot]]

Codecov Report

Merging #707 (1b4f0a3) into main (8105d14) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #707   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         2426      2426           
=========================================
  Hits          2426      2426           

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