doc: Extract net permissions doc
Summary:
Moving the documentation of each flag form the already over-large init.cpp into the net permissions module should clean up the code a bit. Moreover, making the documentation available is also required for an (currently imaginary) setnetpermissions RPC.
Backport of core#19192.
Test Plan:
bitcoind -help
Check the -whitebind option is unchanged.
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9325