UPSTREAM: tracing/histogram: Document hist trigger variables

Update the tracefs README to describe how hist trigger variables
can be created.

Link: https://lkml.kernel.org/r/20211029183339.3216491-4-kaleshsingh@google.com



Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
(cherry picked from commit 6a6e5ef2)

Bug: 146055070
Bug: 145972256
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Change-Id: I8ea0a4c44fe3496d13a09bcf4922e8aa5cdb78ba