[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

Partial backport of Core 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