pybind/ceph_argparse: improve helpstr for CephChoices

Include the field name

Signed-off-by: Sage Weil <sage@newdream.net>

src/pybind/ceph_argparse.py

@@ -844,7 +844,7 @@ def helpstr(self):
             elif self.t == CephPrefix:
                 chunk = str(self.instance)
             elif self.t == CephChoices:
-                chunk = str(self.instance)
+                chunk = f'<{self.name}:{self.instance}>'
             elif self.t == CephOsdName:
                 # it just so happens all CephOsdName commands are named 'id' anyway,
                 # so <id|osd.id> is perfect.