Problem: Using severity names (like "ERROR", "WARN") in functions like
`vim.diganostic.get()` and `vim.diagnostic.config()` is allowed and
tested for many years now. But documentation about it can be clearer.
Solution: Explicitly mention that severity names are allowed in some
situations. Ideally, it would also require updating typing for
`vim.diagnostic.SeverityFilter`, but that looks problematic to do
robustly.
(cherry picked from commit 711f3cc299)
9d66c7828e
