Update description of dangerously_unaliased_fixme directive

Reviewed By: gordyf

Differential Revision: D64254100

fbshipit-source-id: b3719f92942880d183fb2c772d017ac419669942

Author: captbaritone

compiler/crates/relay-schema/src/relay-extensions.graphql

@@ -318,9 +318,17 @@ directive @alias(as: String) on FRAGMENT_SPREAD | INLINE_FRAGMENT
 """
 (Relay Only)
 
-This directive allows users to opt out of validation which enforces that @alias
-be used on all fragment spreads which might not match. It is intended as an
-escape hatch for incremental adoption of enforcing `@alias`.
+**To resolve, replace `@dangerously_unaliased_fixme` with `@alias`**
+
+This fragment spread will only conditionally be fetched, either due to
+`@skip`/@include` or its type condition. Conditionally fetched fragments without
+an `@alias` are unsafe because they don't expose any mechanism for the user to
+check if the data was fetched before using the fragment. In these cases `@alias`
+will materialize the fragment as a nullable named property, ensuring the proper
+null checks are performed.
+
+We now require all such fragment spreads to use `@alias`, and have marked
+existing unsafe spreads with `@dangerously_unaliased_fixme`.
 
 DO NOT ADD NEW USES OF THIS DIRECTIVE.