Clarify HeaderFunc argument is mutable

client/types/startsettings.go

@@ -19,6 +19,7 @@ type StartSettings struct {
 
 	// Optional function that can be used to modify the HTTP headers
 	// before each HTTP request.
+	// Can modify and return the argument or return the argument without modifying.
 	HeaderFunc func(http.Header) http.Header
 
 	// Optional TLS config for HTTP connection.