[avalanche] Document the staking rewards enable flag

Summary: This used to be a hidden flag, make it available via the help.

Test Plan:

bitcoind -help

Check the flag is properly documented.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14626