docs: tracing: Update fprobe documentation

docs: tracing: Update fprobe documentation

Update fprobe.rst for
 - the private entry_data argument
 - the return value of the entry handler
 - the nr_rethook_node field.

Link: https://lkml.kernel.org/r/167526701579.433354.3057889264263546659.stgit@mhiramat.roam.corp.google.com
Cc: Florent Revest <revest@chromium.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>