include_subdomains description missing DNS-only limitation

[Slonser]

The include_subdomains member description does not mention that subdomain policies only generate reports for DNS phase errors.

Step 9 of "Generate a network error report" specifies:

If origin is not equal to policy's policy origin, policy's subdomains flag is include, and report body's phase property is not dns, return null.

This should be documented in the member description itself, not just in the algorithm and Privacy Considerations.

#200