src: add help for NODE_PENDING_DEPRECATION env

PR-URL: nodejs/node#15609
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

src/node.cc

@@ -3899,6 +3899,8 @@ static void PrintHelp() {
          "NODE_PATH                    ':'-separated list of directories\n"
 #endif
          "                             prefixed to the module search path\n"
+         "NODE_PENDING_DEPRECATION     set to 1 to emit pending deprecation\n"
+         "                             warnings\n"
          "NODE_REPL_HISTORY            path to the persistent REPL history\n"
          "                             file\n"
          "NODE_REDIRECT_WARNINGS       write warnings to path instead of\n"