[golang/go] net: improve documentation for net.Dialer.FallbackDelay

### ROOT CAUSE The documentation for `net.Dialer.FallbackDelay` is unclear about its intended use, particularly that it is only used in the context of dual-stack dialing. It fails to explain that this field is ignored when the dialer is not used for dual-stack operations, leading to potential confusion for developers. ### CODE FIX Update the comment for `FallbackDelay` to clarify its purpose and usage: ```go // FallbackDelay is the delay before trying the next dialer in the list of dialers for the dual-stack dialer. // This field is used by the dual-stack dialer (which is created by the DualStack function) to delay between // attempts. It is ignored when the dialer is not used in a dual-stack context. // Must be positive. FallbackDelay time.Duration ``` This change ensures developers understand the field's specific role in dual-stack dialing and that it is not applicable in other scenarios.