tracing/Documentation: Add a section about backup instance

disable tracing with the "traceoff" flag, and enable tracing after boot up.

Otherwise the trace from the most recent boot will be mixed with the trace

from the previous boot, and may make it confusing to read.

Using a backup instance for keeping previous boot data

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It is also possible to record trace data at system boot time by specifying

events with the persistent ring buffer, but in this case the data before the

reboot will be lost before it can be read. This problem can be solved by a

backup instance. From the kernel command line::

  reserve_mem=12M:4096:trace trace_instance=boot_map@trace,sched,irq trace_instance=backup=boot_map

On boot up, the previous data in the "boot_map" is copied to the "backup"

instance, and the "sched:*" and "irq:*" events for the current boot are traced

in the "boot_map". Thus the user can read the previous boot data from the "backup"

instance without stopping the trace.

Note that this "backup" instance is readonly, and will be removed automatically

if you clear the trace data or read out all trace data from the "trace_pipe"

or the "trace_pipe_raw" files.