Fix for the `mask_treatment` parameter documentation in the BoxcarExt… #708

Job	Run time
tests / Load tox environments	6s
tests / Codestyle	12s
tests / Python 3.10 on Linux with oldest supported dependencies	1m 10s
tests / Python 3.10 on Windows with minimal dependencies	1m 44s
tests / Python 3.10 on OSX with minimal dependencies	1m 23s
tests / Python 3.11 on Linux with all dependencies, remote data, and coverage	1m 9s
tests / (Allowed Failure) Python 3.12 on Linux with dev dependencies	1m 6s
	6m 50s

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