Merge #834: Document return type of sign method.

ff72078 Document return type of `sign` method. (Thibaut Le Guilly)

Pull request description:

  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.

  ### Description

  Documents the meaning of the returned value on the wallet `sign` method.

  I've just edited from github so skipped all the checks if that's not ok maybe someone else can update it properly.

ACKs for top commit:
  w0xlt:
    ACK ff72078
  notmandatory:
    ACK ff72078

Tree-SHA512: e0dd52a0af663b88d3fe0a20cb2909b9ba00bcf4f224b4f87cfd53fee0a199a01679dc5cd74aeb440326dfc52d45fa887534138d25df6fa1c8a38678be23204e

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*