doc: document the length requirements of output parameter.
doc: document the length requirements of output parameter.

Summary:
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 <rusty@rustcorp.com.au>

This is a backport of libsecp256k1 [[ https://github.com/bitcoin-core/secp256k1/pull/714 | PR714 ]].

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5389

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