What does this implement/fix? Explain your changes.
Add a deprecation (removed in 0.21) and performance warning for
Any other comments?
I removed the section on approximate neighbors entirely, but I'm having second thoughts about it.
If we go ahead and remove it I'll mention in the relevant PRs (that allow ANN, I think PR #8999 does that) that what we removed can be used as a basis for new documentation.
I would have liked to keep some paragraphs but it feels weird to have general considerations that don't relate to any scikit-learn class.