Add clarifications.

tests/snapshot_tests/snapshot_apps/hot_reloading_app.py

@@ -7,6 +7,9 @@ from textual.widgets import Label
 
 CSS_PATH = (Path(__file__) / "../hot_reloading_app.css").resolve()
 
+# Write some CSS to the file before the app loads.
+# Then, the test will clear all the CSS to see if the
+# hot reloading applies the changes correctly.
 CSS_PATH.write_text(
     """
 Container {