nit: add prdoc for 3370 (#3401)

Did not realize that
https://github.com/paritytech/polkadot-sdk/pull/3370 was indeed
breaking. cc @muraca

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

prdoc/pr_3370.prdoc

+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "Remove `key` getter from pallet-sudo"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Removed the `key` getter function from the sudo pallet. There is no replacement for getting
+      the key currently.
+
+crates:
+  - name: pallet-sudo