DiagnosticUnnecessary semantic tokens are suppressed

[LasseKrarup]

When setting the underline diagnostic settings:

vim.diagnostic.config({
        underline = { severity = vim.diagnostic.severity.ERROR },
}

...the DiagnosticUnnecessary semantic tokens are suppressed.
This causes LSP's to not grey out unused variables.

This is intentional as stated in neovim/neovim#26796
However, it is very unintuitive, so perhaps for this reason it deserves a comment or a value of like underline = { severity = vim.diagnostic.severity.HINT } or underline = true

[rivenirvana]

This is going to be removed in #1550