feat(systemd_lsp): Update functionality and add extra documentation.

lsp/systemd_lsp.lua

@@ -12,9 +12,73 @@
 --- ```
 ---
 --- A language server implementation for Systemd unit files made in Rust.
+---
+--- > [!NOTE]
+--- >
+--- > If you want this LSP to accurately activate for any
+--- > Systemd unit files, use the following settings:
+---
+--- ```lua
+--- vim.filetype.add {
+---   extension = {
+---     -- systemd unit filetypes
+---     service = 'systemd',
+---     socket = 'systemd',
+---     timer = 'systemd',
+---     mount = 'systemd',
+---     automount = 'systemd',
+---     swap = 'systemd',
+---     target = 'systemd',
+---     path = 'systemd',
+---     slice = 'systemd',
+---     scope = 'systemd',
+---     device = 'systemd',
+---     -- Podman Quadlet filetypes
+---     container = 'systemd',
+---     volume = 'systemd',
+---     network = 'systemd',
+---     kube = 'systemd',
+---     pod = 'systemd',
+---     build = 'systemd',
+---     image = 'systemd',
+---   },
+--- }
+--- ```
+
 
 ---@type vim.lsp.Config
 return {
   cmd = { 'systemd-lsp' },
   filetypes = { 'systemd' },
+  ---@param bufnr integer
+  root_dir = function(bufnr, on_dir)
+    local fname = vim.api.nvim_buf_get_name(bufnr)
+
+    local systemd_unit_filetypes = { -- Credit to @magnuslarsen
+      -- systemd unit files
+      '*.service',
+      '*.socket',
+      '*.timer',
+      '*.mount',
+      '*.automount',
+      '*.swap',
+      '*.target',
+      '*.path',
+      '*.slice',
+      '*.scope',
+      '*.device',
+      -- Podman Quadlet files
+      '*.container',
+      '*.volume',
+      '*.network',
+      '*.kube',
+      '*.pod',
+      '*.build',
+      '*.image',
+    }
+
+    local util = require('lspconfig.util')
+
+    on_dir((util.root_pattern(systemd_unit_filetypes))(fname))
+  end,
 }