Documentation: tracing: Add histogram syntax to boot-time tracing

Add the documentation about histogram syntax in boot-time tracing.
This will allow user to write the histogram setting in a structured
parameters.

Link: https://lkml.kernel.org/r/162856127129.203126.15551542847575916525.stgit@devnote2

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