document is_subtype logic

[mr-c]

• Should update Schema Salad specification v1.2 to indicate the old behavior (can only subtype simple classes) and describe the new behavior in Schema Salad spec v1.3
• SpecializeTo should be is_subtype of SpecializeFrom if it isn't checked already, maybe another thing to add for Schema Salad v1.3 spec

Originally posted by @tetron in #741 (review)