DOC: behavior when slicing with missing bounds
 - [x] closes #16917
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff`` (On Windows, ``git diff upstream/master -u -- "*.py" | flake8 --diff`` might work as an alternative.)
 - [ ] whatsnew entry

This documents the behavior discussed in #16917. I don't have time now to produce tests (and I suspect some already exist), so feel free to not close that issue, or to open a new one for tests only.