tracing: Documentation for in-kernel synthetic event API

Add Documentation for creating and generating synthetic events from
modules.

Link: http://lkml.kernel.org/r/734bf8789ff8700000c9acde61a553427910ddb5.1580323897.git.zanussi@kernel.org



Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>