6f2c4fd netinfo: add user help documentation (Jon Atack)

Pull request description:

  This is the help doc commit of bitcoin#20764 without the rest of the PR or anything new since the 0.21.0 branch-off in order to target giving users a -netinfo help doc for 0.21.

  - to test the new help
  ```
  $ ./src/bitcoin-cli -netinfo help
  ```
  - to see the updated short help
  ```
  $ ./src/bitcoin-cli -help | grep -A4 netinfo
  ```

  <details><summary><code>-netinfo</code> help doc</summary><p>

  ```
  $ ./src/bitcoin-cli -netinfo help
  -netinfo level "help"

  Returns a network peer connections dashboard with information from the remote server.
  Under the hood, -netinfo fetches the data by calling getpeerinfo and getnetworkinfo.
  An optional integer argument from 0 to 4 can be passed for different peers listings.
  Pass "help" to see this detailed help documentation.
  If more than one argument is passed, only the first one is read and parsed.
  Suggestion: use with the Linux watch(1) command for a live dashboard; see example below.

  Arguments:
  1. level (integer 0-4, optional)  Specify the info level of the peers dashboard (default 0):
                                    0 - Connection counts and local addresses
                                    1 - Like 0 but with a peers listing (without address or version columns)
                                    2 - Like 1 but with an address column
                                    3 - Like 1 but with a version column
                                    4 - Like 1 but with both address and version columns
  2. help (string "help", optional) Print this help documentation instead of the dashboard.

  Result:

  * The peers listing in levels 1-4 displays all of the peers sorted by direction and minimum ping time:

    Column   Description
    ------   -----------
    <->      Direction
             "in"  - inbound connections are those initiated by the peer
             "out" - outbound connections are those initiated by us
    type     Type of peer connection
             "full"   - full relay, the default
             "block"  - block relay; like full relay but does not relay transactions or addresses
    net      Network the peer connected through ("ipv4", "ipv6", "onion", "i2p", or "cjdns")
    mping    Minimum observed ping time, in milliseconds (ms)
    ping     Last observed ping time, in milliseconds (ms)
    send     Time since last message sent to the peer, in seconds
    recv     Time since last message received from the peer, in seconds
    txn      Time since last novel transaction received from the peer and accepted into our mempool, in minutes
    blk      Time since last novel block passing initial validity checks received from the peer, in minutes
    age      Duration of connection to the peer, in minutes
    asmap    Mapped AS (Autonomous System) number in the BGP route to the peer, used for diversifying
             peer selection (only displayed if the -asmap config option is set)
    id       Peer index, in increasing order of peer connections since node startup
    address  IP address and port of the peer
    version  Peer version and subversion concatenated, e.g. "70016/Satoshi:21.0.0/"

  * The connection counts table displays the number of peers by direction, network, and the totals
    for each, as well as a column for block relay peers.

  * The local addresses table lists each local address broadcast by the node, the port, and the score.

  Examples:

  Connection counts and local addresses only
  > bitcoin-cli -netinfo

  Compact peers listing
  > bitcoin-cli -netinfo 1

  Full dashboard
  > bitcoin-cli -netinfo 4

  Full live dashboard, adjust --interval or --no-title as needed (Linux)
  > watch --interval 1 --no-title ./src/bitcoin-cli -netinfo 4

  See this help
  > bitcoin-cli -netinfo help

  ```
  </p></details>

ACKs for top commit:
  laanwj:
    ACK 6f2c4fd

Tree-SHA512: dd49b1ce65546dacfb8ba9f9d57de0eae55560fd05533cf26c0b5d6ec65bf1de789c3287e90a0e2f47707532fab2fe62919a4192a7ffd58ac8eec18293e9aaeb