docs: update API examples

Author: slhck

docs/advanced/api.md

@@ -3,3 +3,24 @@
 This program has a simple API that can be used to integrate it into other Python programs.
 
 The API docs are [available here](https://htmlpreview.github.io/?https://github.com/slhck/ffmpeg-normalize/blob/master/docs-api/ffmpeg_normalize.html).
+
+## Custom Environment Variables
+
+If you need to pass custom environment variables to ffmpeg (e.g., for setting `LD_LIBRARY_PATH` or other runtime configuration), use the `ffmpeg_env` context manager:
+
+```python
+import os
+from ffmpeg_normalize import FFmpegNormalize, ffmpeg_env
+
+# Create a custom environment with additional variables
+custom_env = os.environ.copy()
+custom_env["LD_LIBRARY_PATH"] = "/custom/lib/path"
+
+# Run normalization with custom environment
+with ffmpeg_env(custom_env):
+    normalizer = FFmpegNormalize()
+    normalizer.add_media_file("input.mp4", "output.mp4")
+    normalizer.run_normalization()
+```
+
+The context manager is thread-safe (uses `contextvars`), so concurrent normalizations in different threads can use different environments.