Some of the built-in kinds have documentation that describes what they are in the Index. For example:
thing (plural things)
Represents anything interactive in the model world that is not a room. People, pieces of scenery, furniture, doors and mislaid umbrellas might all be examples, and so might more surprising things like the sound of birdsong or a shaft of sunlight.
How do I document custom kinds in this way?
Use “specification,” as in:
A weapon is a kind of thing. The specification of weapon is "Represents something with which one player can attack another."
Note that this can be done for actions as well:
Slashing it with is an action applying to one carried thing and one thing. The specification of the slashing it with action is "By this action, an actor can attack another with a bladed weapon."
See ch 24.9 – Extensions in the Index.