docs: Clarify proxy_pass setting in Nginx example

This should help prevent issues like #452 (comment).

Author: Acconut

examples/nginx.conf

@@ -53,6 +53,9 @@ server {
 
     location / {
         # Forward incoming requests to local tusd instance
+        # Note that the proxy's URL does not include a path/URI and not even a trailing slash, so
+        # that Nginx passes the unmodified request URL to tusd.
+        # https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_pass
         proxy_pass http://localhost:8080;
 
         # Disable request and response buffering