doc/trace_analysis.rst: Update intro
doc/trace_analysis.rst: Update intro

* Simplify a bit the example
* Make it clear that systrace has fundamental issues that cannot be
  worked around, so that things may or may not work as expected.

ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa