do not report version variables as unused

Author: Mpdreamz

src/Elastic.Markdown/DocumentationGenerator.cs

@@ -180,7 +180,13 @@ private void HintUnusedSubstitutionKeys()
 	{
 		var definedKeys = new HashSet<string>(Context.Configuration.Substitutions.Keys.ToArray());
 		var inUse = new HashSet<string>(Context.Collector.InUseSubstitutionKeys.Keys);
-		var keysNotInUse = definedKeys.Except(inUse).ToArray();
+		var keysNotInUse = definedKeys.Except(inUse)
+				// versions keys are injected
+				.Where(key => !key.StartsWith("version."))
+				// reserving context namespace
+				.Where(key => !key.StartsWith("context."))
+				.ToArray();
+
 		// If we have less than 20 unused keys, emit them separately,
 		// Otherwise emit one hint with all of them for brevity
 		if (keysNotInUse.Length >= 20)