docs: add root_dir instructions to nvim 0.11 migration guide

[J-MR-T]

When migrating my personal config, I had to manually look for new usages of root_dir in lsp/* (found a good example in lsp/ols.lua), as the function signature for root_dir in vim.lsp.config differs from the old lspconfig root_dir. So I thought some instructions for this in the docs couldn't hurt :).

Arguably, this could be in another part of the docs instead of just the migration guide, as it should be relevant to new users as well. However, if #3768 is solved soon, e.g. if neovim/neovim#33771 gets merged, this will most likely be irrelevant anyway, in favor of wildcards in root_markers; I'll leave it up to you :).

[J-MR-T]

Sorry in case some part of the vim help syntax is off, or my instructions are wrong, I'm mostly just a user ^^