Add release notes for validateaddress Bech32 error detection

doc/release-notes-16807.md

@@ -0,0 +1,6 @@
+Updated RPCs
+------------
+
+- The `validateaddress` RPC now optionally returns an `error_locations` array, with the indices of
+invalid characters in the address. For example, this will return the locations of up to two Bech32
+errors.