docs(splunk_hec sink): update docs re duplicate tags (vectordotdev#15807)

* Update sink docs

Signed-off-by: Stephen Wakely <[email protected]>

* cue fmt

Signed-off-by: Stephen Wakely <[email protected]>

Signed-off-by: Stephen Wakely <[email protected]>

Author: StephenWakely

website/cue/reference/components/sinks/splunk_hec_metrics.cue

@@ -144,5 +144,12 @@ components: sinks: splunk_hec_metrics: {
 
 	telemetry: components.sinks.splunk_hec_logs.telemetry
 
-	how_it_works: sinks._splunk_hec.how_it_works
+	how_it_works: sinks._splunk_hec.how_it_works & {
+		multi_value_tags: {
+			title: "Multivalue Tags"
+			body: """
+				If Splunk recieves a tag with multiple values it will only take the last value specified,
+				so Vector only sends this last value.
+				"""
+		}}
 }