docs: Document implemented clangd extensions (#44308)

Zed currently doesn’t support all protocol extensions implemented by
`clangd`, but it does support two:

- `textDocument/inactiveRegion`
- `textDocument/switchSourceHeader`

Release Notes:

- N/A

---------

Co-authored-by: Kunall Banerjee <hey@kimchiii.space>

docs/src/languages/cpp.md

@@ -158,3 +158,26 @@ You can use CodeLLDB or GDB to debug native binaries. (Make sure that your build
   }
 ]
 ```
+
+## Protocol Extensions
+
+Zed currently implements the following `clangd` [extensions](https://clangd.llvm.org/extensions):
+
+### Inactive Regions
+
+Automatically dims inactive sections of code due to preprocessor directives, such as `#if`, `#ifdef`, or `#ifndef` blocks that evaluate to false.
+
+### Switch Between Source and Header Files
+
+Allows switching between corresponding C++ source files (e.g., `.cpp`) and header files (e.g., `.h`).
+by running the command {#action editor::SwitchSourceHeader} from the command palette or by setting
+a keybinding for the `editor::SwitchSourceHeader` action.
+
+```json [settings]
+{
+  "context": "Editor",
+  "bindings": {
+    "alt-enter": "editor::SwitchSourceHeader"
+  }
+}
+```