Fix options and default in --keyring-provider help (#13110)

The `auto` mode was added a while ago, but the option help was not
updated to reflect this.

Author: radoering

news/a6275be8-84ca-48bf-98dc-1ccb196e7f47.trivial.rst

@@ -260,8 +260,8 @@ class PipOption(Option):
     default="auto",
     help=(
         "Enable the credential lookup via the keyring library if user input is allowed."
-        " Specify which mechanism to use [disabled, import, subprocess]."
-        " (default: disabled)"
+        " Specify which mechanism to use [auto, disabled, import, subprocess]."
+        " (default: %default)"
     ),
 )