Update Elixir docs (#12349)

This PR updates the Elixir docs with a note on how to switch to using
other language server.

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

docs/src/languages/elixir.md

@@ -3,7 +3,37 @@
 - Tree Sitter: [tree-sitter-elixir](https://github.com/elixir-lang/tree-sitter-elixir)
 - Language Server: [elixir-ls](https://github.com/elixir-lsp/elixir-ls)
 
-### Setting up `elixir-ls`
+## Choosing a language server
+
+The Elixir extension offers language server support for `elixir-ls`, `next-ls`, and `lexical`.
+
+`elixir-ls` is enabled by default.
+
+To switch to `next-ls`, add the following to your `settings.json`:
+
+```json
+{
+  "languages": {
+    "Elixir": {
+      "language_servers": ["next-ls", "!elixir-ls", "..."]
+    }
+  }
+}
+```
+
+To switch to `lexical`, add the following to your `settings.json`:
+
+```json
+{
+  "languages": {
+    "Elixir": {
+      "language_servers": ["lexical", "!elixir-ls", "..."]
+    }
+  }
+}
+```
+
+## Setting up `elixir-ls`
 
 1. Install `elixir`: