doc: add txospenderindex release note

Document that newly indexed `txospenderindex` entries use less disk space and that existing indexes remain readable.
Users only need to rebuild the index if they want previously indexed entries rewritten with the smaller marker.

doc/release-notes-35634.md

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+Indexes
+-------
+
+- The transaction output spender index (`-txospenderindex`) now stores one byte
+  less per spender entry for newly indexed blocks. Existing indexes remain
+  compatible and no action is required. To apply the same saving to entries
+  indexed before upgrading, stop the node, delete the
+  `<datadir>/indexes/txospenderindex/` directory, and restart with
+  `-txospenderindex` enabled to rebuild it.