src: document newer values for --unhandled-rejections flag

These values were added in v15.0.0 by nodejs#33475 but were not included in the
`node --help` output.

Also document which value is the default.

Fixes nodejs#37896.

src/node_options.cc

@@ -500,8 +500,12 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
             &EnvironmentOptions::trace_warnings,
             kAllowedInEnvironment);
   AddOption("--unhandled-rejections",
-            "define unhandled rejections behavior. Options are 'strict' (raise "
-            "an error), 'warn' (enforce warnings) or 'none' (silence warnings)",
+            "define unhandled rejections behavior. Options are 'strict' "
+            "(always raise an error), 'throw' (raise an error unless "
+            "'unhandledRejection' hook is set), 'warn' (log a warning), 'none' "
+            "(silence warnings), 'warn-with-error-code' (log a warning and set "
+            "exit code 1 unless 'unhandledRejection' hook is set). (default: "
+            "throw)",
             &EnvironmentOptions::unhandled_rejections,
             kAllowedInEnvironment);
   AddOption("--verify-base-objects",