MAINT/DOC: Rework the *_roots functions and document them.
Merge pull request #3914 from ewmoore/roots_rewrite

Rework the *_roots functions and document them.
TST: sparse: more "known failures" for DOK that don't fail
Merge pull request #3959 from larsmans/dok-fixes

TST: sparse: more "known failures" for DOK that don't fail