argparse: ArgumentOptions.metavar can be an array (#41283)

This is essential for useful docs for nargs >= 2.

https://docs.python.org/dev/library/argparse.html#metavar says this
about metavar and nargs:

Different values of nargs may cause the metavar to be used multiple
times. Providing a tuple to metavar specifies a different display for
each of the arguments.

types/argparse/index.d.ts

@@ -109,7 +109,7 @@ export interface ArgumentOptions {
     choices?: string | string[];
     required?: boolean;
     help?: string;
-    metavar?: string;
+    metavar?: string | string[];
 }
 
 export namespace Const {