(DOC) Clarify resource_type api GET may be for a class, type or nodename

A GET resource_type may request a class, defined type or node by name.
The doc example was just :nodename, which led me briefly astray when I
was querying this endpoint; so I changed it to read
:class_type_or_node_name to make it a little clearer.
Merge pull request #3185 from jpartlow/maint/master/clarify-resource-type-api

(DOC) Clarify resource_type api GET may be for a class, type or nodename