Add unstable prefix for report plaintext field

Author: seuros

proposals/4382-peppered-hash-verification.md

@@ -147,6 +147,7 @@ capabilities.
 
 During development, implementations should use:
 - Field name: `org.matrix.msc4382.verification_hash`
+- Report field: `org.matrix.msc4382.plaintext`
 - Client capability: `org.matrix.msc4382.peppered_hash_verification`
 
 ## Dependencies