Merge #20877: netinfo: user help and argument parsing improvements

7d3343fb8e775527e6afc2b183a62ad76e62347e cli: update -netinfo help doc following the merge of 882ce251 (Jon Atack)
ef614bb408f0547d79bfe9b39278b197e040c683 cli: small -netinfo simplification and performance improvement (Jon Atack)
6b45ef32331dd3efb863d1d2f7405e2665565905 cli: improve -netinfo invalid argument error message (Jon Atack)
3732404afaa15f7a86846327e48cdb9a2027f80c cli: warn in help that -netinfo is not intended to be a stable API (Jon Atack)
7afdd7225842dcff2708dedb8131bb082673f228 cli: enable -netinfo help to run without a remote server (Jon Atack)

Pull request description:

  A few updates, some per IRC discussion today at http://www.erisian.com.au/bitcoin-core-dev/log-2021-01-07.html#l-87 with respect to -netinfo:

  - enable `-netinfo help` to run without a remote server
  - warn in `-netinfo help` that -netinfo is not intended to be a stable API
  - improve the -netinfo invalid argument error message
  - make a performance improvement and simplification I noticed after the merge of #20764
  - update the -netinfo help doc following the merge of #21192
  -----

  How to test manually:  :microscope: :test_tube:  :chart_with_upwards_trend:

  1. check out and build this branch locally; if you need help, don't hesitate to refer to https://jonatack.github.io/articles/how-to-review-pull-requests-in-bitcoin-core#pull-down-the-code-locally or https://jonatack.github.io/articles/how-to-compile-bitcoin-core-and-run-the-tests
  2. while it is compiling, look at the code changes
  3. stop signet (if it is running) with `./src/bitcoin-cli -signet stop`
  4. once the build is completed, run `./src/bitcoin-cli -signet -netinfo help`
  5. the help should be printed even though the signet server is not running
  6. near the top you should see the new warning, "This human-readable interface will change regularly and is not intended to be a stable API" as well as a bit more description about the integer argument values.
  7. start signet with `./src/bitcoind -signet`
  8. test the improved invalid argument error message if you run `./src/bitcoin-cli -signet -netinfo 256` or `./src/bitcoin-cli -signet -netinfo a` (valid values are from 0 to 255)
  9. leave review feedback or `ACK <commit hash>` -- done :beers:

ACKs for top commit:
  michaelfolkson:
    Re-ACK 7d3343fb8e775527e6afc2b183a62ad76e62347e
  pinheadmz:
    RE-ACK 7d3343fb8e775527e6afc2b183a62ad76e62347e

Tree-SHA512: 28c5e9f295ffccba5c2a70faac4987d45f35d4758cf8f10daa767e83212316c4cfc65930e4066f7ad627e9d15b92d43439d1ba9c2f755dfde61885c6a70aa155

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