Update userpass CLI doc for custom path flag (hashicorp#27968)

It's not clear from the get go how to use the userpass auth method with a custom path.

Co-authored-by: Yoko Hyakuna <yoko@hashicorp.com>

website/content/docs/auth/userpass.mdx

@@ -18,6 +18,10 @@ passwords from an external source.
 The method lowercases all submitted usernames, e.g. `Mary` and `mary` are the
 same entry.
 
+This documentation assumes the Username & Password method is mounted at the default `/auth/userpass`
+path in Vault. Since it is possible to enable auth methods at any location,
+please update your CLI calls accordingly with the `-path` flag.
+
 ## Authentication
 
 ### Via the CLI