keyword parameters of functions are not documented

[jurgenvinju]

Describe the bug

• https://docs.rascal-mpl.org/unstable/Rascal/#Declarations-Function
• and also expression syntax for calling functions with keyword parameters is not explained
• note that we do use keyword parameters of functions in the standard library

[jurgenvinju]

Need to check what the status of documenting the dual feature "keyword fields" of ADT constructors.

[jurgenvinju]

• case: https://stackoverflow.com/questions/70324375/changing-the-working-directory-when-using-the-exec-function-in-rascal