simpleperf: update doc for etm/autofdo.

1. Add doc to disable check in AutoFDO.
2. Add a complete example: etm_test_loop.cpp.
3. Add doc for the trace_id mismatch problem.

Bug: 241523910
Test: none
Change-Id: I79d9c552959c9e742d4059e5993f7954e9428db7
3 files changed