doc: Clarify concept of flavor

https://vtkpythonpackage.readthedocs.io/en/latest/Detailed_build_instructions.html#efficiently-building-wheels-for-different-version-of-python

Read the linked Detailed build instructions section, “Efficiently building wheels for different version of Python,” and identify where the term “flavor” is introduced. Clarify that concept in the documentation with enough context for a new reader to understand it; done means the section explains the term without ambiguity.

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