tracing: Explain the trace recursion transition bit better

The current text of the explanation of the transition bit in the trace
recursion protection is not very clear. Improve the text, so that when all
the archs no longer have the issue of tracing between a start of a new
(interrupt) context and updating the preempt_count to reflect the new
context, that it may be removed.

Link: https://lore.kernel.org/all/20211018220203.064a42ed@gandalf.local.home/



Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>