Add a check-doc exception for -greatwallactivationtime
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

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