docs: say that `nil` can be used as a value (#13756)
docs: say that `nil` can be used as a value (#13756)

and improve wording in other ways

Showing that `nil` can be assigned helps to warn readers that variables can be `nil` even after being assigned a value.
dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim