Merge bitcoin/bitcoin#32842: doc: add /spenttxouts to REST-interface.md

dd99ced doc: add `/spenttxouts` to REST-interface.md (Sebastian Falbesoner)

Pull request description:

  Seems like adding the `spenttxouts` endpoint to the REST interface description was forgotten in #32540.

ACKs for top commit:
  maflcko:
    lgtm ACK dd99ced
  pablomartin4btc:
    ACK dd99ced

Tree-SHA512: 9b1da9cbab914664217cc7f8792092e672518ec7f79c7670eb1c54ef94e6cd52b139e1051035ce33ad62b7b74a169e3abc793d1804760787a11a0dc269d26402

Author: fanquake

doc/REST-interface.md

@@ -79,6 +79,13 @@ Responds with 404 if the block doesn't exist.
 Given a height: returns hash of block in best-block-chain at height provided.
 Responds with 404 if block not found.
 
+#### Spent transaction outputs
+`GET /rest/spenttxouts/<BLOCK-HASH>.<bin|hex|json>`
+
+Given a block hash: returns a collection of spent transaction output lists,
+one per transaction in the block.
+Responds with 404 if the block doesn't exist or its undo data is not available.
+
 #### Chaininfos
 `GET /rest/chaininfo.json`