src: add NODE_NO_WARNINGS to --help output

This commit adds a description of the NODE_NO_WARNINGS
environment variable to the --help output. This was overlooked
in 4990212.

Refs: nodejs#10842
PR-URL: nodejs#10918
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

src/node.cc

@@ -3515,6 +3515,7 @@ static void PrintHelp() {
          "                        (will extend linked-in data)\n"
 #endif
 #endif
+         "NODE_NO_WARNINGS           set to 1 to silence process warnings\n"
 #ifdef _WIN32
          "NODE_PATH               ';'-separated list of directories\n"
 #else