docs(stack-encrypt): the source modes are uninhabited and hold no plaintext

Review of #1068 asked whether Borrowed and Owned should derive OpaqueDebug because they stand for a plaintext. They are uninhabited marker enums: no value of either type exists, so their Debug impl can print nothing. The doc now says so, and names where the plaintext actually lives (the description's closure, which has no Debug, and the Pending output).

Author: coderdan

packages/stack-encrypt/src/target/source.rs

@@ -5,11 +5,18 @@
 /// A description's plaintext is handed to it by reference: the default mode,
 /// and the one every [`EncryptFrom`](super::EncryptFrom) declaration runs in.
 /// An operation that consumes the plaintext clones it.
+///
+/// This is a mode, not a value: the enum is uninhabited, so no `Borrowed`
+/// ever exists and it never holds a plaintext. Its `Debug` can print nothing.
 #[derive(Debug)]
 pub enum Borrowed {}
 
 /// A description's plaintext is handed to it by value. A single operation
 /// consumes it without a copy; only fan-out over it needs `Clone`.
+///
+/// Uninhabited like [`Borrowed`]: a mode that never holds the plaintext. The
+/// value itself lives only inside the description's closure, which has no
+/// `Debug`, and in the [`Pending`](super::Pending) output.
 #[derive(Debug)]
 pub enum Owned {}