enable-grpc-gateway is not documented, but may be required

[robinchrist]

Bug report criteria

• This bug report is not security related, security issues should be disclosed privately via etcd maintainers.
• This is not a support request or question, support requests or questions should be raised in the etcd discussion forums.
• You have read the etcd bug reporting guidelines.
• Existing open issues along with etcd frequently asked questions have been checked and this is not a duplicate.

What happened?

When running etcd via config file, the enable-grpc-gateway may be required (see #12093).

When running via CLI args, gRPC gateway is implicitly enabled. When running via config file, it is not.

What did you expect to happen?

Ideally, grpc gateway should always be enabled by default (to match the behaviour of config file to the behaviour of running via CLI options)

If this is not possible, the flag should be documented at least

How can we reproduce it (as minimally and precisely as possible)?

See #12093

Anything else we need to know?

No response