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