Restrict to Model
By default, Link Properties can contain anything. This usually isn't what
you want, though -- most often, you're looking for Instances of a specific Model. For example,
if you specify the Stylesheet Property, you only want to give Stylesheets as options:
it would be meaningless to have the Stylesheet contain something like a recipe or a to-do list.
So this is a meta-Property: when you create a Thing Property, you can add this to say
exactly which Things it can contain. It is strongly recommended that you set this on all Thing Properties
you create -- it makes them easier to use, and tends to prevent confusing errors.
Note that this is only enforced loosely, and you can't absolutely count upon this restriction
always being true. But used properly, it will steer folks in the right direction.
