Add help text for uahfstarttime command line option
Summary: Adds a help text that also shows default value.
Test Plan:
make check
Run 'src/bitcoind --help' and check uahfstartttime description.
Reviewers: #bitcoin_abc, deadalnix, sickpig, awemany, Mengerian, CCulianu, dagurval
Reviewed By: #bitcoin_abc, deadalnix, dagurval
Subscribers: dagurval
Differential Revision: https://reviews.bitcoinabc.org/D320