Update gen-manpages instructions in release-process.md

Summary: Add instructions that work for the case of out-of-tree builds

Test Plan: tried generating manpages using the instruction

Reviewers: jasonbcox, Fabien, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2284