tracing.rst neglects to document syntax of trace-events files

[Markus Armbruster]

Almost[*] all trace-events files start with this line:

    # See docs/devel/tracing.rst for syntax documentation.

But I can't find syntax documentation there.


[*] Exceptions:
    $ git-ls-files \*/trace-events| xargs fgrep -L '# See 
docs/devel/tracing.rst for syntax documentation.'
    ebpf/trace-events
    hw/core/trace-events
    hw/hyperv/trace-events
    hw/mips/trace-events
    hw/net/can/trace-events
    hw/nubus/trace-events
    hw/nvme/trace-events
    hw/remote/trace-events
    hw/sh4/trace-events
    hw/ssi/trace-events
    target/arm/hvf/trace-events
    target/riscv/trace-events
    target/s390x/kvm/trace-events