text: Title: Document that `Get Variable Value` and `Variable Should (Not) Exist` do not support named-argument syntax
  Body: Hi,
  
  I noticed that I am not able to pass `default` argument for `Get Variable Value` from `BuiltIn` as keyword argument. I.e. the following does not provide expected result: `${result}  BuiltIn.Get Variable Value  ${MY_VARIABLE}  default=My Default`
  
  One would expect `${result}` to have `My Default` as return if `${MY_VARIABLE}` does not exist. Instead `default=My Default` is returned. 
  
  ![get_variable_value_example](https://github.com/user-attachments/assets/9fe63afc-67a7-4c82-811d-f63c547cac23)
  
  I didn't find any notion of this functionality anywhere and if the keyword works as expected, I would assume that this was mentioned in the documentation.
  
  This was tested with version 7.0.