Merge bitcoindevkit/bdk#834: Document return type of `sign` method.

ff720780950e010cb13af7def1d2226841ca3f97 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 https://github.com/bitcoindevkit/bdk/pull/834/commits/ff720780950e010cb13af7def1d2226841ca3f97
  notmandatory:
    ACK ff720780950e010cb13af7def1d2226841ca3f97

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