doc: add release note for #25957 (fast wallet rescan)

doc/release-notes-25957.md

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+Wallet
+------
+
+- Rescans for descriptor wallets are now significantly faster if compact
+  block filters (BIP158) are available. Since those are not constructed
+  by default, the configuration option "-blockfilterindex=1" has to be
+  provided to take advantage of the optimization. This improves the
+  performance of the RPC calls `rescanblockchain`, `importdescriptors`
+  and `restorewallet`. (#25957)