give context around @acmePeriodicAuto location block. thanks javier!

examples/nginx.conf

@@ -69,8 +69,13 @@ http {
     ssl_certificate data:$dynamic_ssl_cert;
     ssl_certificate_key data:$dynamic_ssl_key;
 
-    # Check certificate validity each minute
+    # `js_periodic` must be in a location {} block, so use a named location to
+    # avoid affecting URI space.
+    # From https://nginx.org/en/docs/http/ngx_http_core_module.html#location
+    #   The “@” prefix defines a named location. Such a location is not used for a
+    #   regular request processing, but instead used for request redirection.
     location @acmePeriodicAuto {
+      # Check certificate validity each minute
       js_periodic acme.clientAutoMode interval=1m;
     }