Documentation spellchecker

The automated manual build runs the spellchecker over the documentation sources and records every word it does not know. Not all of them are misspellings: technical terms, author names, and syntax the checker cannot recognize are collected in doc/utils/sphinx-config/false_positives.txt and belong there rather than in the text. A genuine typo is fixed in the .rst source itself. Both are reproduced locally with make spelling in the doc directory.

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