improve help doc for -usecashaddr argument

Summary: The activation date without a year is a bit confusing. This was introduced in v0.16.2

Test Plan:

$ src/bitcoind -h

Options:

  -usecashaddr
       Use Cash Address for destination encoding instead of legacy base58
       addresses (default: 1)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18801