doc: add release notes for 29415

doc/release-notes-29415.md

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+P2P and network changes
+-----------------------
+
+- Normally local transactions are broadcast to all connected peers with
+  which we do transaction relay. Now, for the `sendrawtransaction` RPC
+  this behavior can be changed to only do the broadcast via the Tor or
+  I2P networks. A new boolean option `-privatebroadcast` has been added
+  to enable this behavior. This improves the privacy of the transaction
+  originator in two aspects:
+  1. Their IP address (and thus geolocation) is never known to the
+     recipients.
+  2. If the originator sends two otherwise unrelated transactions, they
+     will not be linkable. This is because a separate connection is used
+     for broadcasting each transaction. (#29415)