fgraph: Add comment to describe ftrace_graph_get_ret_stack

The ret_stack should not be accessed directly via the curr_ret_stack
variable on the task_struct. This is because the ret_stack is going to be
converted into a series of longs and not an array of ret_stack structures.
The way that a ret_stack should be retrieved is via the
ftrace_graph_get_ret_stack structure, but it needs to be documented on how
to use it.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>