Explain Resource narrative missing in examples page - N-Infra

[lmckenzi]

(from https://jira.hl7.org/browse/FHIR-18543)

The examples page for resources that inherit directly from Resource should note that they cannot have narrative because they inherit from resource. (It's confusing to have a page that says it displays the narrative with an empty narrative and no explanation of why.)

Suppress the message saying that this is narrative and instead say "No narrative can be present for [resource type] instances." or something like that.