docs(dockerls): document ignoreMultilineInstructions setting

Document how the ignoreMultilineInstructions settings for dockerls can
be configured.

That the settings key must be "docker" instead of "dockerls" makes it
non-intuitive and harder to figure out.

Sources:
- https://github.com/rcjsuen/dockerfile-language-server?tab=readme-ov-file#language-server-settings
- https://github.com/rcjsuen/dockerfile-language-server/blob/9e3f8580f711e54a1de0351631baeac49665d6f3/src/server.ts#L330-L344

lua/lspconfig/server_configurations/dockerls.lua

@@ -6,6 +6,15 @@ return {
     filetypes = { 'dockerfile' },
     root_dir = util.root_pattern 'Dockerfile',
     single_file_support = true,
+    settings = {
+      docker = {
+        languageserver = {
+          formatter = {
+            ignoreMultilineInstructions = true,
+          },
+        },
+      }
+    },
   },
   docs = {
     description = [[