Document IsA Container

[tajmone]

• Add Admonition/Note in §2.4. Introduction to the Language » The SYNTAX about use of IsA Container in the example.
  • Add xref to a section where special use of IsA with Container is explained in more detail (currently to be created).

See also #45 on IsA Container.

In §2.4. Introduction to the Language » The SYNTAX we encounter the first usage example of IsA Container:

It is also possible to restrict the types of the parameters:

Syntax
  put_in = 'put' (obj) 'in' (cont)
    Where obj IsA object
      Else "You can't put that into anything."
    And cont IsA Container
      Else "Nothing fits inside that."

This restricts the parameter obj to being an instance inheriting from the object class (as opposed to an actor for example) and the parameter cont to a Container (an instance with the Container property).

It's worth adding a note/admonition there about the special use of IsA with a non-class.