Fix argparse rST parsing of help messages (#14014)

Thanks!

lib/spack/llnl/util/argparsewriter.py

@@ -10,9 +10,10 @@
 import sys
 
 
-class ArgparseWriter(object):
+class ArgparseWriter(argparse.HelpFormatter):
     """Analyzes an argparse ArgumentParser for easy generation of help."""
     def __init__(self, out=sys.stdout):
+        super(ArgparseWriter, self).__init__(out)
         self.level = 0
         self.out = out
 
@@ -48,7 +49,7 @@ def _write(self, parser, root=True, level=0):
         def action_group(function, actions):
             for action in actions:
                 arg = fmt._format_action_invocation(action)
-                help = action.help if action.help else ''
+                help = self._expand_help(action) if action.help else ''
                 function(arg, re.sub('\n', ' ', help))
 
         if root: