[backport#16239] wallet/rpc: use static help text
[backport#16239] wallet/rpc: use static help text

Summary:
Always show the same help topic regardless of wallet flags, and explain that something is not always available, rather than runtime-modifying the help output.

https://github.com/bitcoin/bitcoin/pull/16239/commits/3d2ff379131a01e4e9f9648b150e806104a23795

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Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/16239 | PR16239]]

Test Plan:
  ninja
  ./src/bitcoind -regtest -daemon
  ./src/bitcoin-cli -regtest help sendtoaddress
  ./src/bitcoin-cli -regtest help getbalance
  ./src/bitcoin-cli -regtest help listunspent

check help output looks spiffy

  test_runner.py wallet_avoidreuse

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6486

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