Merge #17713: doc: Add release notes for 17447

fa4b656e97 doc: Add release notes for 17447 (MarcoFalke)

Pull request description:

  Stolen from https://github.com/bitcoin/bitcoin/pull/17447#issuecomment-553475914

ACKs for top commit:
  promag:
    ACK fa4b656e97.
  laanwj:
    ACK fa4b656e97

Tree-SHA512: 5d281c0a85e75c9fae8885faf0e4a2ca4e4f73788f3d214ca65c7c891203a7435cc77fe3046e2d7e3e2226d96c547005f1d970e768d6cd82423f575e07881431

doc/release-notes.md

@@ -104,6 +104,10 @@ Low-level changes
 Tests
 -----
 
+- It is now an error to use an unqualified `walletdir=path` setting in the config file if running on testnet or regtest
+  networks. The setting now needs to be qualified as `chain.walletdir=path` or placed in the appropriate `[chain]`
+  section. (#17447)
+
 - `-fallbackfee` was 0 (disabled) by default for the main chain, but 0.0002 by default for the test chains. Now it is 0
   by default for all chains. Testnet and regtest users will have to add `fallbackfee=0.0002` to their configuration if
   they weren't setting it and they want it to keep working like before. (#16524)