docs: Update developing extension docs for updating specific submodule (#42548)

Release Notes:

- N/A

docs/src/extensions/developing-extensions.md

@@ -109,6 +109,18 @@ git submodule init
 git submodule update
 ```
 
+## Update Your Extension
+
+When developing/updating your extension, you will likely need to update its content from its submodule in the extensions repository.
+To quickly fetch the latest code for only specific extension (and avoid updating all others), use the specific path:
+
+```sh
+# From the root of the repository:
+git submodule update --remote extensions/your-extension-name
+```
+
+> Note: If you need to update all submodules (e.g., if multiple extensions have changed, or for a full clean build), you can run `git submodule update` without a path, but this will take longer.
+
 ## Extension License Requirements
 
 As of October 1st, 2025, extension repositories must include a license.