Document no_spa setting

I used the same doc comment as found in the code

Author: janhohenheim

Trunk.toml

@@ -40,6 +40,8 @@ addresses = ["127.0.0.1"]
 port = 8080
 # Open a browser tab once the initial build is complete.
 open = false
+# Whether to disable fallback to index.html for missing files.
+no_spa = false
 # Disable auto-reload of the web app.
 no_autoreload = false
 # Disable error reporting