dev docs: clarify internal and external token helpers (#13014)

* dev docs: clarify internal and external token helpers

* Add docs for DefaultTokenHelper

command/util.go

@@ -14,6 +14,7 @@ import (
 )
 
 // DefaultTokenHelper returns the token helper that is configured for Vault.
+// This helper should only be used for non-server CLI commands.
 func DefaultTokenHelper() (token.TokenHelper, error) {
 	return config.DefaultTokenHelper()
 }