document multi-value headers as undefined

For #213

apis/v1alpha2/httproute_types.go

@@ -343,12 +343,18 @@ type HTTPHeaderMatch struct {
 	// Name is the name of the HTTP Header to be matched. Name matching MUST be
 	// case insensitive. (See https://tools.ietf.org/html/rfc7230#section-3.2).
 	//
-	// If multiple entries specify equivalent header names, only the first entry
-	// with an equivalent name MUST be considered for a match. Subsequent
+	// If multiple entries specify equivalent header names, only the first
+	// entry with an equivalent name MUST be considered for a match. Subsequent
 	// entries with an equivalent header name MUST be ignored. Due to the
 	// case-insensitivity of header names, "foo" and "Foo" are considered
 	// equivalent.
 	//
+	// When a header is repeated in an HTTP request, it is
+	// implementation-specific behavior as to how this is represented.
+	// Generally, proxies should follow the guidance from the RFC:
+	// https://www.rfc-editor.org/rfc/rfc7230.html#section-3.2.2 regarding
+	// processing a repeated header, with special handling for "Set-Cookie".
+	//
 	// +kubebuilder:validation:MinLength=1
 	// +kubebuilder:validation:MaxLength=256
 	Name string `json:"name"`