Clarify ActionLink URL generation documentation

[mahdiaghtaee]

{PR title}

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Summary of the changes (Less than 80 chars)

Description

Summary of the changes (Less than 80 chars)

Clarify ActionLink XML documentation for null protocol and host behavior.

Description

Updates the XML documentation for ActionLink to match the actual implementation.

When protocol or host is null, ActionLink uses the current request's scheme and host instead of requiring both parameters to be non-null.

Fixes #42949

[dotnet-policy-service]

Thanks for your PR, @mahdiaghtaee. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.