tracing: Add hist trigger comments for variable-related fields

Add a few comments to help clarify how variable and variable reference
fields are used in the code.

Link: http://lkml.kernel.org/r/ea857ce948531d7bec712bbb0f38360aa1d378ec.1545161087.git.tom.zanussi@linux.intel.com



Acked-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>