Update docs for mainnet merge release (#3494)

## Proposed Changes

Update the merge migration docs to encourage updating mainnet configs _now_!

The docs are also updated to recommend _against_ `--suggested-fee-recipient` on the beacon node (https://github.com/sigp/lighthouse/issues/3432).

Additionally the `--help` for the CLI is updated to match with a few small semantic changes:

- `--execution-jwt` is no longer allowed without `--execution-endpoint`. We've ended up without a default for `--execution-endpoint`, so I think that's fine.
- The flags related to the JWT are only allowed if `--execution-jwt` is provided.

validator_client/src/cli.rs

@@ -131,8 +131,9 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
         .arg(
             Arg::with_name("suggested-fee-recipient")
                 .long("suggested-fee-recipient")
-                .help("The fallback address provided to the BN if nothing suitable is found \
-                           in the validator definitions or fee recipient file.")
+                .help("Once the merge has happened, this address will receive transaction fees \
+                       from blocks proposed by this validator client. If a fee recipient is \
+                       configured in the validator definitions it takes priority over this value.")
                 .value_name("FEE-RECIPIENT")
                 .takes_value(true)
         )