Update manpages for 0.18.6

Summary: As per doc/release-process.md

Test Plan: Read and make sure it makes sense

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: jasonbcox, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2279