As the max stack tracer algorithm is not that easy to understand from the
code, add comments that explain the algorithm and mentions how
ARCH_FTRACE_SHIFT_STACK_TRACER affects it.

Link: http://lkml.kernel.org/r/20190806123455.487ac02b@gandalf.local.homeSuggested-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>