tracing: Add tracing/error_log Documentation

Move most of the hist trigger extended error documentation to
ftrace.rst and expand on it to fully document tracing/error_log.

Link: http://lkml.kernel.org/r/c5d53c8f643ef6844d6ad8d0200c116936730b01.1554072478.git.tom.zanussi@linux.intel.com

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

Documentation/trace/histogram.rst

@@ -199,20 +199,8 @@ Extended error information
 
   For some error conditions encountered when invoking a hist trigger
   command, extended error information is available via the
-  corresponding event's 'hist' file.  Reading the hist file after an
-  error will display more detailed information about what went wrong,
-  if information is available.  This extended error information will
-  be available until the next hist trigger command for that event.
-
-  If available for a given error condition, the extended error
-  information and usage takes the following form::
-
-    # echo xxx > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger
-    echo: write error: Invalid argument
-
-    # cat /sys/kernel/debug/tracing/events/sched/sched_wakeup/hist
-    ERROR: Couldn't yyy: zzz
-      Last command: xxx
+  tracing/error_log file.  See Error Conditions in
+  :file:`Documentation/trace/ftrace.rst` for details.
 
 6.2 'hist' trigger examples
 ---------------------------