Add schema modeling guidance for enum values as UPPER_SNAKE_CASE

[jack-berg]

UPPER_SNAKE_CASE alternative to #445.

Advantage of upper snake case is that enums stick out very clearly.

Disadvantage is that more types are impacted:

• lower_snake_case impacts 2 types, both of which are experimental
• UPPER_SNAKE_CASE impacts 7 types, one of which is experimental, 6 of which are on stack to stabilize