The following additions to the tracing documentation are included:
1. Move "stderr" backend documentation to top-level and out of "simple"
backend. Include hints on when this backend is useful.
2. Document the "simple" backend thread-safety limitation.
3. Document the "dtrace" backend for SystemTap.
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
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