docs: tracing: Add stacktrace filter command

Add a description of stacktrace filter command.
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/ftrace.rst

@@ -2987,6 +2987,9 @@ The following commands are supported:
   command, it only prints out the contents of the ring buffer for the
   CPU that executed the function that triggered the dump.
 
+- stacktrace:
+  When the function is hit, a stack trace is recorded.
+
 trace_pipe
 ----------