Document missing deprecation guides

[locks]

From https://github.com/thoov/ember-2-legacy/blob/master/config/environment.js:

• _ENABLE_UNDERSCORE_ACTIONS_SUPPORT (private API)
• _ENABLE_RESOLVER_FUNCTION_SUPPORT Guide
• _ENABLE_REVERSED_OBSERVER_SUPPORT Source
• _ENABLE_ROUTER_RESOURCE Source
• _ENABLE_CURRENT_WHEN_SUPPORT Source
• _ENABLE_DEPRECATED_REGISTRY_SUPPORT Guide
• _ENABLE_STRING_FMT_SUPPORT PR / Guide
• _ENABLE_FREEZABLE_SUPPORT Guide
• _ENABLE_PROPERTY_REQUIRED_SUPPORT

[thoov]

_ENABLE_RESOLVER_FUNCTION_SUPPORT (https://emberjs.com/deprecations/v2.x/#toc_registry-resolver-as-function) & _ENABLE_FREEZABLE_SUPPORT (https://emberjs.com/deprecations/v1.x/#toc_ember-freezable). Already exists so we can mark them complete.

[thoov]

_ENABLE_DEPRECATED_REGISTRY_SUPPORT (https://emberjs.com/deprecations/v2.x/#toc_ember-application-registry-ember-applicationinstance-registry) already exists

[thoov]

_ENABLE_PROPERTY_REQUIRED_SUPPORT should be added to deps 1.12 (emberjs/ember.js#10668)

[cibernox]

_ENABLE_CURRENT_WHEN_SUPPORT is documented in #3154

[btecu]

This can probably be closed.