Merge #714: doc: document the length requirements of output parameter.

4b48a431060948dc5e29aa590d646a72aa138968 doc: document the length requirements of output parameter. (Rusty Russell)

Pull request description:

  It's subtle, since it is actually only touched by hashfp (though
  we assert it's non-NULL), but give explicit advice in the default
  case.

  Signed-off-by: Rusty Russell <[email protected]>
ACKs for top commit:
  jonasnick:
    ACK 4b48a431060948dc5e29aa590d646a72aa138968
  real-or-random:
    ACK 4b48a431060948dc5e29aa590d646a72aa138968 diff inspection

Tree-SHA512: d6bedb495e46b27ac9b558e77d814884d782ea78569a2296688eccf374bc880d13846546ad449c2a677865cf6ed56fcbc8be58c21f9daca5084831074e20d769