Add a check-doc exception for -greatwallactivationtime

Summary: Will prevent arc lint to fail with undocumented argument until the fork.

Test Plan:

arc lint --everything

Should return no error

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2387