perf docs: Correct and clarify jitdump spec

Specification claims latest version of jitdump file format is 2. Current
jit dump reading code treats 1 as the latest version.

Correct spec to match code.

The original language made it unclear the value to be written in the
magic field.

Revise language that the writer always writes the same value. Specify
that the reader uses the value to detect endian mismatches.

Signed-off-by: Steve MacLean <Steve.MacLean@Microsoft.com>
Acked-by: Stephane Eranian <eranian@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Brian Robbins <brianrob@microsoft.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Eric Saint-Etienne <eric.saint.etienne@oracle.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Keeping <john@metanate.com>
Cc: John Salem <josalem@microsoft.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Tom McDonald <thomas.mcdonald@microsoft.com>
Link: http://lore.kernel.org/lkml/BN8PR21MB1362F63CDE7AC69736FC7F9EF7800@BN8PR21MB1362.namprd21.prod.outlook.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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