Allow wallet files not in -walletdir directory

Remove restriction that -wallet filenames can only refer to files in the
-walletdir directory.

doc/release-notes.md

@@ -63,6 +63,15 @@ RPC changes
 
 - The `fundrawtransaction` rpc will reject the previously deprecated `reserveChangeKey` option.
 
+External wallet files
+---------------------
+
+The `-wallet=<path>` option now accepts full paths instead of requiring wallets
+to be located in the -walletdir directory. When wallets are located in
+different directories, wallet data will be stored independently, so data from
+every wallet is not mixed into the same <walletdir>/database/log.??????????
+files.
+
 Credits
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