Document return type of sign method.

Small thing but I had to look up the code to see what the returned boolean was about, thought it might be good to have in the docs.

src/wallet/mod.rs

@@ -1084,7 +1084,7 @@ where
     }
 
     /// Sign a transaction with all the wallet's signers, in the order specified by every signer's
-    /// [`SignerOrdering`]
+    /// [`SignerOrdering`]. This function returns the `Result` type with an encapsulated `bool` that has the value true if the PSBT was finalized, or false otherwise.
     ///
     /// The [`SignOptions`] can be used to tweak the behavior of the software signers, and the way
     /// the transaction is finalized at the end. Note that it can't be guaranteed that *every*