Document voluntary exiting validators with public key.

[rolandtyler]

Use voluntary-exit subcommand for voluntary exit of validators added via REST API.
The voluntary-exit subcommand can be used with a validator public keys filter option to support the validators added via REST API: --public-keys-filter.
Example: teku voluntary-exit --beacon-node-api-endpoint=<ENDPOINT>[,<ENDPOINT>...]... --data-validator-path=<PATH> --include-keymanager-keys=<BOOLEAN> --validator-keys=<KEY_DIR>:<PASS_DIR> | <KEY_FILE>:<PASS_FILE> --public-keys-filter=<PUBKEY>[,<PUBKEY>...]...
To include validator keys managed via keymanager APIs, the option --include-keymanager-keys could be set to true (The default value is set to false).

See Consensys/teku#6250

[alexandratran]

Already fixed by #401