docs(alloy_ls): include tips for setting filetype on *.als files (#3561)

lua/lspconfig/configs/alloy_ls.lua

@@ -12,6 +12,24 @@ return {
 https://github.com/AlloyTools/org.alloytools.alloy
 
 Alloy is a formal specification language for describing structures and a tool for exploring them.
+
+You may also need to configure the filetype for Alloy (*.als) files:
+
+```
+autocmd BufNewFile,BufRead *.als set filetype=alloy
+```
+
+or
+
+```lua
+vim.filetype.add({
+  pattern = {
+    ['.*/*.als'] = 'alloy',
+  },
+})
+```
+
+Alternatively, you may use a syntax plugin like https://github.com/runoshun/vim-alloy.
 ]],
   },
 }