[avalanche] Show the avalanche options in the help

Summary:
They used to be hidden because of the experimental status of avalanche. Some of them will be removed in follow-ups.
Also improve the avacooldown/maxavalancheoutbound documentation by adding the unit/default value.

Test Plan:

./bitcoind -help

Check there is an avalanche category and the options are documented.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11912