tracing: Add histogram-design document

Add a new Documentation/trace/histogram-design.rst file describing the
ftrace histogram low-level design, meant to help developers trying to
understand the internals when extending or making use of the hist
triggers for higher-level tools.

This documentation refers to the hist_debug files implemented by
'tracing: Add hist_debug trace event files for histogram debugging' so
users wishing to try out the test examples here should make sure
CONFIG_HIST_TRIGGERS_DEBUG is enabled.

Link: http://lkml.kernel.org/r/256b29c3274bb89a10157c4a8d1a8bce7e74849e.1585941485.git.zanussi@kernel.orgSigned-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>