Low-level docs for v3 model are now in place

5/6 of the way there!

datamodel/low/v3/server_variable.go

@@ -4,6 +4,13 @@ import (
 	"github.com/pb33f/libopenapi/datamodel/low"
 )
 
+// ServerVariable represents a low-level OpenAPI 3+ ServerVariable object.
+//
+// ServerVariable is an object representing a Server Variable for server URL template substitution.
+// - https://spec.openapis.org/oas/v3.1.0#server-variable-object
+//
+// This is the only struct that is not Buildable, it's not used by anything other than a Server instance,
+// and it has nothing to build that requires it to be buildable.
 type ServerVariable struct {
 	Enum        []low.NodeReference[string]
 	Default     low.NodeReference[string]