doc: add release notes for 20867

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

doc/release-notes-20867.md

@@ -0,0 +1,11 @@
+Wallet
+------
+
+- We now support up to 20 keys in `multi()` and `sortedmulti()` descriptors
+  under `wsh()`. (#20867)
+
+Updated RPCs
+------------
+
+- `addmultisigaddress` and `createmultisig` now support up to 20 keys for
+  Segwit addresses.