docs(bevy_ecs): clarify how to configure Res/ResMut validation error … (#24315)

# Objective

Fixes #23770

The documentation for `Res`, `ResMut`, `NonSend`, and `NonSendMut`
stated
that validation failures "can be configured to do nothing or warn once"
but never explained how to actually configure it.

## Solution

Replaced the misleading sentence with accurate information pointing
users
to `If<T>` for silently skipping the system, and clarified that
`Option<T>` is for when you want to handle the missing resource
yourself.

## Testing

No testing is required. These are documentation-only changes.