Mention why tasks may have 'unknown' as name.

doc/chapters/perf-feedback.texi

@@ -285,6 +285,9 @@ visualizing open-source tool}.  It is possible to open the
 % vite paje.trace
 @end example
 
+To get names of tasks instead of "unknown", fill the optional @code{name} field
+of the codelets, or use a performance model for them.
+
 By default, all tasks are displayed using a green color. To display tasks with
 varying colors, pass option @code{-c} to @code{starpu_fxt_tool}.