Profile: mention `kill -s SIGUSR1 julia_pid` for Linux (#56441)

currentlu this route is mentioned in docs
https://docs.julialang.org/en/v1/stdlib/Profile/#Triggered-During-Execution


but missing from the module docstring, this should help users who have
little idea how to "send a kernel signal to a process" to get started

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Co-authored-by: Ian Butterworth <i.r.butterworth@gmail.com>