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 PR714.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

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