chore: update escrow to use PrivateImmutable (#4942)

Working on #4940 I
found that this contract uses `PrivateSet` (aka `Set`), when
`PrivateImmutable` would actually be a much better fit. The escrow was
reading set notes without nullifying them because it doesn't ever remove
notes from the set (and hence all notes that were ever created are
always valid). This can be very confusing for a beginner, and was a bad
example to showcase.

I also removed the docs decorators since we don't ever reference them.

noir-projects/noir-contracts/contracts/escrow_contract/src/main.nr

@@ -1,40 +1,31 @@
 // Sample escrow contract that stores a balance of a private token on behalf of an owner.
 contract Escrow {
-    use dep::aztec::prelude::{AztecAddress, EthAddress, FunctionSelector, NoteHeader, NoteGetterOptions, PrivateContext, PrivateSet};
+    use dep::aztec::prelude::{AztecAddress, EthAddress, FunctionSelector, NoteHeader, PrivateContext, PrivateImmutable};
 
     use dep::aztec::{context::{PublicContext, Context}, oracle::get_public_key::get_public_key};
 
     use dep::address_note::address_note::{AddressNote, ADDRESS_NOTE_LEN};
 
     struct Storage {
-        owners: PrivateSet<AddressNote>,
+        owner: PrivateImmutable<AddressNote>,
     }
 
     // Creates a new instance
-    // docs:start:constructor
     #[aztec(private)]
     #[aztec(initializer)]
     fn constructor(owner: pub AztecAddress) {
-        let this = context.this_address();
-
-        // Create a new note and add it to the owners set.
-        let mut note = AddressNote::new(owner, this);
-
-        // Insert the owner into storage
-        storage.owners.insert(&mut note, true);
+        let mut note = AddressNote::new(owner, owner);
+        storage.owner.initialize(&mut note, true);
     }
-    // docs:end:constructor
 
-    // Withdraws balance. Requires that msg.sender is registered as an owner.
+    // Withdraws balance. Requires that msg.sender is the owner.
     #[aztec(private)]
     fn withdraw(token: AztecAddress, amount: Field, recipient: AztecAddress) {
         let this = context.this_address();
         let sender = context.msg_sender();
 
-        // We don't remove note from the owners set. If a note exists, the owner and recipient are legit.
-        let options = NoteGetterOptions::new().select(0, sender.to_field(), Option::none()).select(1, this.to_field(), Option::none()).set_limit(1);
-        let notes = storage.owners.get_notes(options);
-        assert(notes[0].is_some(), "Sender is not an owner.");
+        let note = storage.owner.get_note();
+        assert(note.address == sender);
 
         let selector = FunctionSelector::from_signature("transfer((Field),(Field),Field,Field)");
         let _callStackItem = context.call_private_function(