Revise secrets-dir flag in the VC (#5480)

* Update docs on secrets-dir

* Hidden secrets-dir flag

* Remove conflicts_with

* Restore description

* make cli

* Update book/src/validator-management.md

validator_client/src/cli.rs

@@ -73,7 +73,6 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     key. Defaults to ~/.lighthouse/{network}/secrets.",
                 )
                 .takes_value(true)
-                .conflicts_with("datadir")
         )
         .arg(
             Arg::with_name("init-slashing-protection")