Merge bitcoin/bitcoin#31277: doc: mention descriptorprocesspsbt in psbt.md

ebb6cd8 doc: mention `descriptorprocesspsbt` in psbt.md (Sebastian Falbesoner)

Pull request description:

  Noticed that the `descriptorprocesspsbt` RPC call is currently not documented anywhere.

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Tree-SHA512: 47f3c8693dfc79bf2d38176ee015efe8c1a1539b95f9e0eb3db7693e4ef39222ad2b5cb44d55042ca1eb4288ba5e486068f66c054ff0a59e989b6cdc8e4461f6

Author: fanquake

doc/psbt.md

@@ -67,6 +67,10 @@ hardware implementations will typically implement multiple roles simultaneously.
   input a PSBT, adds UTXO, key, and script data to inputs and outputs that miss
   it, and optionally signs inputs. Where possible it also finalizes the partial
   signatures.
+- **`descriptorprocesspsbt` (Updater, Signer, Finalizer)** is a node RPC that takes
+  as input a PSBT and a list of descriptors. It updates SegWit inputs with
+  information available from the UTXO set and the mempool and signs the inputs using
+  the provided descriptors. Where possible it also finalizes the partial signatures.
 - **`utxoupdatepsbt` (Updater)** is a node RPC that takes a PSBT and updates it
   to include information available from the UTXO set (works only for SegWit
   inputs).