Merge bitcoin/bitcoin#28373: doc: Add example of mixing private and public keys in descriptors

24b67fa doc: Add example of mixing private and public keys in descriptors (Anton A)

Pull request description:

  closes: #27414

ACKs for top commit:
  achow101:
    ACK 24b67fa
  alfonsoromanz:
    Re ACK 24b67fa

Tree-SHA512: 8c063f23199ac0ff35909f786a5b0de1b4a9b15d1e93bdcdac10cb4bd2002c12e99b6fb1c2e56d16971e7622b67d910b79088429df92c48279be2d7797049911

Author: achow101

doc/descriptors.md

@@ -243,7 +243,18 @@ Often it is useful to communicate a description of scripts along with the
 necessary private keys. For this reason, anywhere a public key or xpub is
 supported, a private key in WIF format or xprv may be provided instead.
 This is useful when private keys are necessary for hardened derivation
-steps, or for dumping wallet descriptors including private key material.
+steps, for signing transactions, or for dumping wallet descriptors
+including private key material.
+
+For example, after importing the following 2-of-3 multisig descriptor
+into a wallet, one could use `signrawtransactionwithwallet`
+to sign a transaction with the first key:
+```
+sh(multi(2,xprv.../84'/0'/0'/0/0,xpub1...,xpub2...))
+```
+Note how the first key is an xprv private key with a specific derivation path,
+while the other two are public keys.
+
 
 ### Compatibility with old wallets