docs: update docs to point to derivation_of_spk

updated docs to point `derivation_of_spk` function to find whether a `KeychainKind` exits  for the
given `script`

fix bitcoindevkit#1042

crates/bdk/src/wallet/mod.rs

@@ -815,6 +815,8 @@ impl<D> Wallet<D> {
  }
 
  /// Return whether or not a `script` is part of this wallet (either internal or external)
+ /// Use `derivation_of_spk` function to find the `KeychainKind` for the `script`, or
+ /// None if there's no associated `Keychain`.
  pub fn is_mine(&self, script: &Script) -> bool {
  self.indexed_graph.index.index_of_spk(script).is_some()
  }