Update the documentation for dropbool

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Description

Fixes the documentation which described the behaviour of dropbool incorrectly. dropbool() currently drops the values where the value in the second series set IS zero instead of, as described, dropping those where the value IS NOT zero. To avoid a breaking change which would give the function the more intuitive behaviour from its name, this only updates the documentation.

We should consider fixing this with a breaking change in a future release.

Fixes #2496

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

How has this been tested?

• Query dropbool(series("foo=bar", 0, 1, 1, 1, 2, 1, 3, 1), series("", 0, 1, 1, 2, 2, 3, 3, 2) != 2) returns {"0": 1, "2": 1}
• Query dropbool(series("foo=bar", 0, 1, 1, 1, 2, 1, 3, 1), series("", 0, 1, 1, 2, 2, 3, 3, 2)) returns {"1": 1, "2": 1, "3": 1}

The behaviour is the inverse of the previously described behaviour. The description is fixed with this PR.

Checklist:

• [x] This contribution follows the project's code of conduct
• [x] This contribution follows the project's contributing guidelines
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules