[core#19724 9/9][doc] Follow developer notes, add comment about missing default.

Summary:
This concludes backport of core#19724 [9/9]
https://github.com/bitcoin/bitcoin/pull/19724/commits/eb1c5d090f4ef844dabc1e8bf3864c1afe1c791c

Depends on D9777

Test Plan: Proof-reading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9778