doc: Extract net permissions doc #19192

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.

Can be tested by calling ./contrib/devtools/gen-manpages.sh before and after and observing the difference.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #19191 (net: Extract download permission from noban by MarcoFalke)
• #19180 (refactor: Replace RecursiveMutex with Mutex in Shutdown() by hebasto)
• #19176 (refactor: Error message bilingual_str consistency by laanwj)
• #18972 (WIP: net: Add blockfilters white{bind,list} permission flag by luke-jr)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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Rebased to solve merge conflict and reduce diff by +1-1

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$ bitcoind -regtest -h | grep -A8 whitebind
  -whitebind=<[permissions@]addr>
       Bind to given address and whitelist peers connecting to it. Use
       [host]:port notation for IPv6. Allowed permissions: bloomfilter
       (allow requesting BIP37 filtered blocks and transactions), noban
       (do not ban for misbehavior), forcerelay (relay transactions that
       are already in the mempool; implies relay), relay (relay even in
       -blocksonly mode), mempool (allow requesting BIP35 mempool
       contents). Specify multiple permissions separated by commas
       (default: noban,mempool,relay). Can be specified multiple times.
--

It might be helpful to append an example at the end of the help.

It might be helpful to append an example at the end of the help.

I'll keep it as is for now to minimize the rendered diff. If deemed important, this can be done in a follow-up, but I think -whitebind=<[permissions@]addr>, with the (default: noban,mempool,relay) already serve as a good example.

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