Add explanatory comment to NGINX_ENABLE_CERTBOT_CHALLENGE key in .env.example (#7154)

docker/.env.example

@@ -657,6 +657,7 @@ NGINX_KEEPALIVE_TIMEOUT=65
 NGINX_PROXY_READ_TIMEOUT=3600s
 NGINX_PROXY_SEND_TIMEOUT=3600s
 
+# Set true to accept requests for /.well-known/acme-challenge/
 NGINX_ENABLE_CERTBOT_CHALLENGE=false
 
 # ------------------------------