DOC: Document other required options

Eric Larson requested to merge larsoner/vtk:docs into master

Document the other required options.

In theory the Makefile / build process could be updated to either turn these on automatically, or raise an error if there are incompatible options. But this seems like the most expedient solution, and it works for me on Linux and macOS at least.

Backport: release

Edited by Ben Boeckel

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