Add non-normative explanation of moduleHandler.referrer

[kriskowal]

With #77, we no longer thread referrer and do not have a need to thread the referrer. However, every practical implementation will need to use a referrer property on module handler objects in order to resolve relative import specifiers. To that end, we’ve agreed at the September 21, 2022 SES call to add a non-normative block explaining how virtual modules can resolve relative import specifiers.