Add release notes for (a)sync z_sendmany changes

doc/release-notes.md

@@ -21,6 +21,15 @@ RPC Changes
     also available for testnet and regtest nodes, in which case it does not
     return end-of-service halt information (as testnet and regtest nodes do not
     have an end-of-service halt feature.)
+- `z_sendmany` will no longer select transparent coinbase when "ANY\_TADDR" is
+  used as the `fromaddress`. It was already documented to do this, but the
+  previous behavior didn’t match. When coinbase notes were selected in this
+  case, they would (properly) require that the transaction didn’t have any
+  change, but this could be confusing, as the documentation stated that these
+  two conditions (using "ANY\_TADDR" and disallowing change) wouldn’t coincide.
+- Several `z_sendmany` failures have moved from synchronous to asynchronous, so
+  while you should already be checking the async operation status, there are now
+  more cases that may trigger failure at that stage.
 
 [Deprecations](https://zcash.github.io/zcash/user/deprecation.html)
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